Are typically seen as a damage class, and in some content this is primarily true, however in early content, and indeed late content Mages are much more of a utility class than a DPS class due to their very strong control abilities. Mages appear to dominate early on as the King and Queen of Crowd Control for a reason, Sheep. This is a reliable (with max hit rating it has something like a 1% chance to break each tick, but the exact mechanics appear to be a bit fuzzy) effective and long duration control method that can be reapplied at any time.
Mages are not solely restricted to this role, typically coming in as the third highest DPS, and second highest AoE DPS class (though potentially the highest in high mobility fights). Their DPS role however is more complex than a Warlock's, relying on a rotation of abilities to apply debuffs / buffs to maximise DPS. Certain abilities, primarily snares / roots, can be harmful to the Tank's ability to Tank.
Mages offer two primary benefits to a party, firstly is an intelligence buff, typically applied to all Mana using classes to increase the buffer / DPS time without reliance on external sources such as Mana pots (and yes even a few hundred Mana can make a huge difference in rotations if it pushes you over a repetition point in your cycle, interrupting a cycle is a lot more harmful than it seems). This buff is however not the major benefit Mages bring to a group.
The table, or creation of stacks of water and food, assists a party greatly, it significantly lowers the running costs of a group at the expense of annoying the Mage to make it. Typically for a level 70 party the food / water costs for a Paladin Tank can hit 8G (4 stacks of Food / Water is doable in longer instances when you don't have the ability to chain pull groups easily) / instance while running, assuming at least the Healer also makes use of similar numbers that is 16G per instance saved. Pre-level 70 this ability creates stacks individually and can be annoying to apply, however creating a minimum of 2 stacks of Food / Water for the Tank and Healer (of course no Water for a Warrior or Druid :P), and Food / Water for the DPS as appropriate.
Mages should always have their Mana gems created and ready for use, this allows them to maintain a much better DPS rotation without additional External resources, effectively doubling the MP5 gained from a potion (since a Mana Gem can be used to offset the 2 minute cooldown). Many Mage specs are very mana intensive, and their itemisation does not support Int / Spirit typically (or rather it is sacrificed by choice) meaning reliance on a Shadow Priest or Shaman which cannot be guaranteed in non-raid situations (and often not there).
Mages are an interesting class to assign groups too, they are 100% consumers of group buffs, however provide no buffs in return. Mages typically prefer to be in groups with Shadow Priests (for the mana return), or a Shaman (Wrath of Air Totem for bonus spell damage), however both of these are more beneficial to Warlocks (at least in the short term while Mana Gems last as Life Tapping significantly drops a Warlock's DPS).
As such Mages should typically be assigned in a similar way to Hunters, filling in slots in Shadow Priest, then Shaman, then Hunter groups (Beast Mastery) where possible. Ideally shifting Mages into a Shadow Priest group in place of Warlocks later in the fight (once the 3 Mage Gems have been used), should act as a DPS boost as the Warlocks can maintain their DPS at a reduced rate, however the Mages remain viable DPS rather than running out of Mana, thought typically this should only come up in longer fights, or chain fights such as Zul'aman trash.
Mages are thankfully easy to Bless, for short intensive fights like trash Kings should swap position with Wisdom. As with Hunters some Mages prefer Kings to Wisdom, listen to your Mages as they should know their spec better than you and some specs prefer one over the other.
Blessing of Salvation
Blessing of Wisdom
Blessing of Kings
Blessing of Light (if multiple Paladins present)
Blessing of Sanctuary
Blessing of Might
Mages provide one main form of Crowd Control (which is typically the gold-standard of crowd control), Sheep, however can additionally slow, root and kite targets, however only the former is in regular usage.
Sheep is a 1 minute long buff (with no cooldown, so it can be applied at any time), with a minimal chance to be broken (1% per tick roughly for a Hit capped Mage). The Sheeped target moves in a small area, and is vulnerable to all DPS, being broken by AoE, DoTs or any similar effect. Sheep cannot be reapplied effectively while the target is DoTted, doing so wastes Mana and DPS time, if this situation occurs the target needs to be controlled in another manner.
As with Hunters this control should be applied first, typically a Tank pulls and as the mobs start moving sheep is applied, doing it half way towards the Tank such that the Mage can remain behind the Tank / out of melee range of Tanked targets and still reapply it. Awareness is important, while Sheep can be applied repeatedly (chain sheeping) this is typically not a useful benefit as the Sheep has the same chance to resist and break the Sheep irrespective of when cast, it simply reduces the chance the target gets free without a Sheep being cast on it. An aware Mage could be applying DPS to another target rather than performing the "chain sheep lol" manoeuvre.
One situation Sheep can be used in as a different form of control (while the target is tanked or under the effect of DoTs / AoE) is to interrupt a channelled effect such as the Arcane Whirlwinds cast by enemies in the Tempest Keep instances The Eye, and The Botanica. This interrupts the cast effectively (some are immune to interrupt, but not to control), and the Tank / DPS's next attack will break the sheep giving minimal interruption except to a typically massive Raid / Party damage ability.
Control can also be applied using tools such as Frost Nova, Cone of Cold and disorientate effects allowing Mages to effectively Kite targets. In small areas such as Shattered Halls a Mage can be more effective at this role than a Hunter due to being able to Blink through the target reducing the chance of being hit. These control abilities should not be underestimated as they present at least 2 and potentially more abilities (for example Dragon's Breath from a Fire Mage) to control a large group of targets for a short while. These abilities can be used very well in conjunction with a Paladin tank to gain initial threat on targets that would otherwise move quickly through a Consecration by stunning them inside the consecration area.
Combat and Utility:
Mages come in 3-4 main flavours, Fire, Frost, Arcane+Fire/Frost. Typically Fire is a crit-based spec, with a massive DPS boost on targets below 20% health, Frost is more reliant on freezing a target (though has talents that allow instant refresh on all cooldowns making it ideal as a PvP build) and Arcane+ is a highly Mana intensive, high threat, high damage build. From a Tank's perspective this makes little difference overall, they all do respectable damage when played correctly and have access to their control abilities.
In combat Mages should be expected to lose 2-10 global cooldowns per minute for control abilities due to Sheep being broken, or being reapplied, on a Boss fight their cooldowns can be more strictly applied to DPS, however Spell Steal and Counter spell should take precedence to DPS when required, as with Tanking the focus is not on damage and topping the meters but rather on making sure that your utility is correctly applied to the situation as that Counter spell or Spell Steal is likely saving the Tank a lot more hurt than killing the target 1 global cooldown earlier would have done. In situations such as Zul'aman Spell Stealing the debuffs from the Flamecasters can result in massive increases in damage, so its not all doom and gloom.
Mages have an ability Evocate that stops them moving / DPSing / Controlling for a short period however massively increases the Mana regeneration for that period allowing them to recover from a Mana drain, or simply being in a sub-ideal group that cannot provide an external Mana source such as a Shadow Priest or Shaman.
Mages have a potentially complete threat drop in Invisibility, allowing them to become inactive for a short period as their threat drops, and eventually fully dropping their threat. Mages typically do not like using this ability due to requiring to remain inactive while it is in progress, however with threat sensitive fights such as Void Reaver, Gurtogg Bloodboil or Teron Gorefiend the lost DPS time is less valuable than being able to really push DPS for the remaining time.
Both of these abilities go against the idea of maintaining steady DPS, however as with all things its better to be in a sensible position for the fight than to go all out and find yourself short in an emergency. Take the time needed to use these abilities at the right time and don't run empty because the situation when you need it may not be appropriate to allow these their full effectiveness.
Area of Effect:
Mages are the second main AoE class, having three different AoE spells, Arcane Explosion (a 10 yard - melee range - spell with a damage cap), Blizzard (their Frost AoE, second most efficient damage wise) and a fire version. Yes I am ignoring the last one, if a Mage casts it its a waste of a cooldown, its much less efficient in terms of damage than the other two even on a Fire Mage (extensive testing in Hyjal, Mages doing this typically got 50-70% of the DPS of Mages doing the other two, might be wrong but my personal opinion is this is a bad plan). While a Mage is unlikely to match the Damage output of Seed of Corruption using these abilities there is no other class that can match this damage potential (other than perhaps a paladin with > 100 targets on them, as Consecration has no damage cap :P, but that situation doesn't exist in game).
Mage's AoE often has secondary functionality such as slow, freeze or disorientates and should be applied carefully in situations where this might cause hardship to the Tank. Care should of course be taken to watch aggro on AoE pulls, and a tool such as Omen in AoE mode will handle this beautifully, remember that the threat in an AoE pull is actually the lowest threat on any target in the pull rather than the highest so watch the meter carefully, a Paladin on a high number of targets will still threat cap you massively as they have to rely on consecration and reactive threat, and cannot easily target through all targets in a pull (though again situations such as this are rarer, and the damage cap on AoE abilities makes it unlikely to be a major issue).
Roots and Snares:
There is also an AoE CC, Frost Nova. This ability is great on pulls that can be killed fast (before it runs out), as an emergency save when a tank dies, or as an emergency button when a Healer / DPS is in trouble while moving. The last part is vital... while moving... Frost Nova locks the mobs in place, but does not stop them attacking, if not ranged they will attack the closest target to them. This means that a Tank cannot gain aggro on the target without being close for the duration of the Nova (they may have aggro, but its being ignored till the mob can move). This means players not moving out of range will die horribly, additionally to Tanks are typically moving to stop the AI spreading mobs into our back arc (can only dodge, parry and block in the front 180 degrees), this means if the mobs are frozen and we are required to move for some reason they will likely enter the rear arc in a way we cannot control. This should be a last ditch ability, using it in any other situation is likely to annoy the Tank as it fully removes control of the fight from us.
Ice Nova and Roots should not be applied except in an emergency, while most Tanks won't notice, and indeed won't realise how annoying it is as its simply a "fact of life", and most Mages without a Tank-alt feel its great, its not. Losing control of a pull and restricting what the Tank can do is a bad thing to do on any pull, in the typical AoE situations this is used in losing control can turn a situation with manageable damage and threat generation into something that will threaten the Tank and Stress the Healer. Thats not to say that you can't use it, or that using it at the right time is a bad thing, but rather than using it in all situations is more likely to be a bad thing than good, as with Hunters you can tell the difference between a good Mage and a bad one through their use of these abilities, a good one supports the Tank using them, a bad one gets you killed.
While Warlocks are typically static DPS classes, by preference they do not move to maximise their global cooldowns spend on damage, Mages can maintain damage abilities on the move, and indeed often prefer to move allow the use of short range cone or AoE effects. This does not mean that moving is a good thing for a Mage, strictly it tends to reduce their damage as standing still casting high damage spells (in a similar way to Warlocks) gives maximal use of time. But being fluid gets you into a good state of mind to use Blink, and lets you use abilities such as Dragon's Breath or Cone of Cold which would otherwise just look pretty.
A Mage being mobile must be concerned with the positioning and movement of the Tank and group and predict movements, often applying a snare or damage to an off-target can result in them becoming a target and being unable to be helped by a Tank. Controlled use of Blink allows Mages to reduce fall damage, avoid snares, environmental damage and similar. While a typical 5 man instance will give you few opportunities to test this its a very useful skill as it allows rapid movement and the ability to survive otherwise deadly situations (blinking out of fires for example on Supremus, Archimonde... actually many bosses).
Spell Steal, Counter Spell and Dispelling:
I can't really say a lot about these abilities, their use is situational, however very important, and a Mage that cannot use these abilities in combat, and more so know which abilities to Steal or Counter is a liability in fights which demand them. Spell Steal is typically the more situational, coming up in High King Maulgar, the Illidari Council, Zul'aman and a few other spots with well defined buffs that are required.
Counter Spell is much more of an art form, the spell lock out makes the caster move towards the current aggro target (good if the Tank has aggro, bad if that Tank is you) so can be used to effectively pull Casters into AoE and melee range. Its rarely bad to counter a Spell since it will reduce the incoming damage significantly however its likely to go unnoticed and unthanked, however doing it and making the difference between Tank life and death is a major thing (I like being alive tbh, its more fun than waiting for resses :P).
Dispelling is a very important Mage ability (and part of why they get dragged into Sunwell kicking and screaming), they are one of only two classes that can remove curses (the other being Druids, but doing so pops them out of Tree form so typically requires a Boomkin to perform). This is a minor benefit typically in the early stages of raiding, however becomes VITAL in later stages, if the curse isn't removed it causes more damage and raid hardship than anything else as its persistent. Learn to decurse, get an addon and be aware of this role, we don't need a dead dps when a live one will work.
Tier 4 Level:
This level of raiding and experience benefits Mages a lot, Paladins Tanks (and Warriors / Druids) have not yet reached the Gear level where they can Tank instances without CC and removing Healing targets from pulls (or MC / high damage) is very beneficial).
At this stage expect to make full use of your abilities, there are instances with environmental damage (Blink), Control (Sheep, Novas), AoE, Curses, and even High King Maulgar himself, who to Tank the Mage essentially requires a Mage with Spell Steal (at later gear levels the pull can be close to solo tanked, however its not really recommended as the Mage hurts).
Mages should perform very well on Damage at this level and gain good advances in gear through the Tailoring items giving them a very effective head start on raw damage.
Tier 5 Level:
In terms of instances rather than Raids at this level and above, the primary reason to bring a Mage over another DPS class (since any good Tank in this level of gear can AoE tank every instance in the game, there is no pull in SSC/TK as of 2.4 that cannot be AoE Tanked though not doing so may be easier) is the creation of a Mage Table (well as well as Curses, AoE, Survival... :P). Again your abilities see a lot of use with Counter Spell being used to setup Kael'thas Sunstrider to pull to the Tank (remember the spell lock out and go beat the Tank plan). Typically control is less of an issue at this level, however many guilds prefer to control 2-4 mobs on each pull to make it simpler and less risky. Again Mage Control is preferred to most other classes here due to its reliability and ability to be reapplied instantly.
At this gear level a Mage's DPS is on par with other main DPS classes and the Control requirements on the instances are much reduced allowing them to spend more time performing their DPS role. At this stage the lack of survivability of Mages in tailoring gear may become more apparent and acquiring a set of PvP or high Stamina gear is recommended for some fights and later Tiers.
Tier 6 Level:
In Tier 6 content (and Tier 6.5) Mages performing their secondary roles becomes vital, controlling Curses and important enemies becomes vital to a pull while their DPS drops compared to that of Warlocks and Rogues due to the greater Synergy these classes have.
Many fights at this level will press the Mages on threat, or survival (Bloodboil has an annoying habit of selecting Mages as his target, and alas they are the only class that does not have a self-heal, avoidance or similar, and Ice Block wipes the raid). Be careful and use your abilities to the maximum, at this stage in the game no Raid Leader should need to tell you to Counter Spell or Dispel it should simply be part of the role in the same way a Tank doesn't allow a free target to run around (unless its about to be Sheeped, in which case we cannot hit it).
As with Hunters a Mage is rarely a bad choice for a slot at this level, their utility is still very high, and their DPS still impressive. Try to maximise your effectiveness with group setup (Shamans, Shadow Priests, Boomkin, Hunters) to really get DPS up, and co-operate on Spec to maximise group DPS / application and reapplication of Scorch.
Raid and Party Responsibilities:
It has been said by multiple Mages that they do not like dropping a Table or Portalling people because it uses reagents, it should be a nice addition to a group however not something that is expected of them. After all its not like a table really gives anything to the group, its not a buff or something that can make the bosses die faster.
This attitude is wrong, a Paladin that does not buff the group hurts its capabilities, a Warlock that refuses to Soulstone a wipe-recovery class or refuses to make health stones is also wrong, and likely would not be tolerated. While the creation of a Table or Portals does appear to not be in the same league (direct combat effectiveness) it is directly related to the expenditure of the group (8G was a typical figure for me in early Heroics, where I had to drink between every pull, reapply buffs etc to make sure I was at full Mana to maximise my threat for the pull), and to the speed at which they can go through an instance. Ensuring the raid or party has a table means that there is no excuse to not have full Mana and Health on each pull, it means everyone can take part in that 10-15s break without worrying about wasting food and water (ever start to eat/drink, get attacked and have to restart?), in short it makes the run much more pleasant to be in because its one fewer worry.
This is primarily a personal view, and especially given that in many instances the level of gear I have, and my regular Healers / DPS have is such that we can 3 man instances. In short having the gear to do these instances without control, and indeed without a full party means that we can afford to be picky and kick people who don't play nice, my repair bill is high irrespective of wipes or not because I take damage, if we wipe my repair will be higher than a cloth class. Every class has benefits and should apply these to a party, it doesn't matter if its the worst PuG in the world, you still apply the 30 minute Paladin buffs that use reagents because it makes it easier for the group. Its part of personal responsibility really, and taking responsibility to maximise what ever benefits you can for the group, you rarely see a Tank asking to be recompensed for the armour repairs they pay irrespective of wipes, or a Paladin asking for the 1S per blessing he had to cast so the cost shouldn't be an issue.
While I haven't directly kicked someone for failing to make a table, I have asked for a Mage that would make a Table (he got 7 badges of Justice, a Primal Nether and 3 Primal Water's for his trouble). Being nice and providing those additional benefit that your class has is what makes instances go well, people trying their best irrespective of what their best is. Never go into an instance thinking its routine and dull, that its not worthy of trying because doing so might dull you to the point that you make a mistake in something that does count.