This class is highyl interesting, it is the most fluid of all the hybrid classes in that it can perform any of its roles to some extent without a gear change or talent respec in between changing those roles. This is not to say its effective, a Feral Druid isn't going to be the best Healer in the world, however a spot heal even if its weak, can make a massive difference when needed. Druids offer a very versatile class to a tank, in their Healer form they have the ability to keep HoTs on many targets and so keep multiple people standing through reasonable AoE damage, in their Feral forms they do good damage, have ok to excellent mitigation and can act as an off-Tank as needed. Finally the "Boomkin" form they are an excellent boost to a spell caster heavy party, however can have significant Threat isssues as they lack threat scaling talents commonly found in most other damage builds.
Druids provide a small variety of Buffs to the party, primarily Mark of the Wild (with group and individual versions) which boosts all statistics and resistances, and thorns (primarily a Tank buff) that does damage to attackers giving a threat boost.
Most Druids don't really know they have Crowd Control, however they have several useful abilities, firstly Roots, this ability (usable outside at the moment) will lock a target in place for a few seconds, and is not broken by damage. This means it can be used to tie up a running target for the Tank, or it can be used pre-emptively on targets which should not be allowed to get close such that the ranged DPS can full out attack the target gaining a few seconds before it approaches the group.
Secondly there are the abilities which function on Beasts, that is Soothe Beast, and Sleep, Sleep triggers combat and otherwise performs like Sheep, typically it is less reliable as Druids other than Boomking will not approach the Hit Rating cap and so will get a larger number of resists. The first is a very situational ability, it reduces the Aggro range of a target significantly for a 15s. This means that if a target is positioned correctly it can be used to skip pulls, such as the first pair of bears in Zul'aman. Both of these abilities are highly situational, however if used correctly are as powerful as Sheep.
Combat and Utility:
Druids have a lot of utility in combat, obviously the biggest is their primary roles which I cover below, however the ability to switch between these in a clutch is a major benefit (and a last minute tranquility or heal has saved me more than once), and can be counted on, Druids know their forms and should try and maximise their potential at any time. Next we have the two reasons we really love Druids, Innervate and Combat Res, the first of these is an ability which increases mana regeneration by 500% for a short period of time, since Regeneration is based on Spirit this is best cast on Priests, then other caster classes and Healers, and finally in a last ditch on a Paladin, Paladins do not stack Spirit so gain very little from this. Typically this is used as an emergency tool on the Healer, though in long fights that are DPS sensitive it can be moved to them.
The Combat Res is a life saver, it is usable once every 30 minutes, and can only be used while the Druid is alive (yes, this means if you are Ressed and the Druid dies it was wasted, pick a time when there is no splash damage and accept it, then get back into the fight). This ability is again typically used to restore party or raid balance and to get a key player / class back into the action, typically you will remain dead for a period until the fight can be gauged as winnable or not, and if it looks good you will be brought back in, typically in the order of Healer > Highest DPS > DPS Least Likely to Die > Tank. Actually the last target on that list is really there as a joke, in anything but a fight with a tauntable boss (Nalarokk, Halazzi, Brutallus, Kalcegos, etc), or phase swaps with Aggro resets (Reliquery of Souls, Hydross the Unstable) do not waste it on a Tank, we will not build sufficient threat in a short period to regain the Aggro position, nor will we add significant buffs to the group compared to getting a Healer or DPS back in the fight. This ability is normally not in the Druid's control, but rather the party leader or the Raid Leader's, that is to say typically do not take the initiative and just use it, ask, or wait for someone to ask otherwise if a key class goes down and it was used on a less viable target it hurts the raid (and for a wipe situation we would like it for the next attempt instead).
Druid Healers are essentially Heal over Time based (HoT), they stack Buffs on the target that restore health over time, this makes them the ideal Healer to deal with regular damage spikes because the next Heal is only a tick away, and requires no interaction further from a healer (so no cancelling a Heal and frantically casting). In their Tree of Life form the Druid gives bonus healing to their group, thus stacking a Druid Healer within the Tank group is often seen as a very useful tactic, though they may get shifted out of this to allow for a better threat group (such as the additon of a Feral Druid). Direct Heals cause the Druid to leave the Tree of Life form and the benefits it granted them, this means that where possible the Tank should ensure that there will not be a requirement for massive continual spike healing (such is more suited to a Paladin with efficient low cast time heals) as it is highly inefficient to force the Druid to leave this form. Druid Healers do however have a very effective fast healing combo of Nature's Swiftness > Regrowth > Swiftmend making the Regrowth an instant cast. There are many tricks to Druid healing, I won't cover them as thats a different post, but don't worry about a Druid not being able to keep you up through a damage spike.
From a Tank's perspective this is the most interesting form, Cat form allows for good dps and dots (bleeds) to be applied to the target, yet they retain the majority of single target tanking skills as the melee combat and tanking talent trees are combined. This means that a Feral Druid can (though remember their armour is no where near the same as in their Tank gear, nor is their health) shift into Dire Bear form and tank 1-2 mobs if needed. This is essentially a great backup Crowd Control form, or can be used to allow you to shift "safe" enemies to the Druid while you deal with a new pickup. Druids in this talent spec are very valuable and often highly proactive in switching forms to provide safety for a Healer or other DPS class, typically it is best to run with their form changing and let them perform pickups as needed, as a form of Crowd Control a backup Tank is unmatched.
In terms of raid utility Feral Druids make the ideal 3rd+ Tank and off-Tank as they can perform good DPS while not tanking, and maintain a good rage generation while off-tanking in a way Warriors and Paladins find hard to match. As their avoidance will be strictly lower than a Warrior or Paladin in most content they do not suffer from a soft-avoidance-cap for Rage / Mana regeneration meaning they can tank even lower levels of content acceptably in high end gear that would push a Paladin or Warrior to a very low threat threshold.
The Balance form (Boomkin) is a Caster DPS type which boosts the other spell casters significantly, they should be assigned to a caster group where possible, however are unfortuantely rare as they at current suffer from Mana and Threat control problems.
Tier 4 Level:
Tier 4 content provides Druids with a lot of their potential, from a non-raid Tank's perspective this is where having the ability to off-Tank or spot-any role as needed is really vital to the party. Druids come into their own and learn to rapidly switch, control and recognise situations in fights at this level. Typically the lack of indoor / non-beast crowd control is a penalty to very early Tier 4 instances, however as the progression into the tier continues and less than full CC is utilised a Druid provides a very solid stepping point between the "Tank, Healer and 3 CC" and "Tank, Healer and 2CC / 1CC" range of ability levels.
In raiding content at least one Druid is beneficial for the Combat Resurrection at a minimum as it allows for longer learning attempts and recovery from simple mistakes. The ability to maintain HoTS or adapt to any role makes the Druid a solid choice for any instance or raid. The flexibility on Gruul and Magtheridon which require a variety in Tank numbers makes them the ideal choice in the third-fifth Tank slots without sacraficing as much raid versatility as you would taking more than two Protection spec Tanks.
Tier 5 Level:
Non-raid content at this level can typically be run with limited or no crowd control, and as such the versatility of the Drudi comes into play again, no major pros or cons to taking a Druid beyond the obvious combat res again.
Raiding again makes great use of Druids at this level, many encounters start to introduce high raid wide splash damage in fights such as Leotheras the Blind, Al'ar, Void Reaver or even Lady Vashj, the ability of a Druid Healer to maintain multiple targets and cover spike damage quickly makes them ideal for at least one slot in this tier level if not more. Once again the off-Tank off-DPS capability of Druids shines and makes them ideal in the third-fourth tank slots in this tier, typically there is not a fifth tank in this tier level. As with lower tier content Druids tend to be pushed towards their off-roles due to the inflexibility of the Protection specced Tanks, there is no fight that cannot be tanked by a Druid at this level with perhaps the exception of Kael'thas Sunstrider due to the lack of a way out of the third Pyroblast.
Tier 6 Level:
Tier 6 content is quite harsh to Druids, dropping the fourth tank slot in most fights and with a preference for Warriors in many of the boss-level encounters. This is not to say that a Druid Tank is not needed, again the flexibility to change their role makes the third Tank slot ideally suited to a Druid since many of the fights are close to single Tank affairs. This tier continues to be beneficial to involve at least one Druid healer, as spike damage becomes more common and small splash damage very likely (though quite predictable so highly benefitting from HoT type healing).