The Tank has a unique and interesting role in most raids and instances, they tend to take on the role of leader, being responsible for the tactics employed, the marking and control of each pull within, as such they have a perspective on each class and how it should be applied to their level of content. This role though is often stressful and error prone, a Tank is unlikely to know your class inside and out, nor to make decisions 100% correctly each time, in fact they are likely to care far more about your classes generalities than your specifics, that you can control a mob for the duration of the pull is more important than whether that target is made immobile, or is simply juggled between two dps classes until it dies. This is the fine art Tanks apply to a party, providing the expertise and knowledge of instances and pulls such that people that know better can make the low level decisions, simply, we don't care how you do it, as long as you can do it.
Of course our control and experience matter little once the fight starts we tend to lose the fine oversight of the fight we had from the static pull position and become more concerned with achieving a full threat rotation, watching our health and ensuring that our active threat rotation is within the limits of our rage or mana regeneration capabilities. This means that our plan must survive contact with the enemy (in contrast to Sun'Tzu's), and that we must see enough and learn enough from people talking to us during the pull to make decisions as to what needs to be altered and what we can safely add to our "tanked" pool. In short, once we pull we often need the DPS and Healer roles to go onto automatic, with only the occasional suggestions offered, however when we give these its rarely a "in 5 minutes could you" situation but rather a "if you don't my life in the next 30s is going to get so annoying". This means that each class needs to have the intuition as to what the Tank beleives you do, and what the tank themselves is going to do, the latter is a lot harder, but the former can be summarised because as I said above, we don't care how you do it, as long as you do it, and talk to us.
The first thing to consider in a generic way is crowd control, that is any ability that takes one or more enemies from the pull, and removes them from the need to be tanked. This can take any form, a Mage's Sheep, a Hunter Trap, or a Warrior Hamstringing an enemy and running around with it chasing him, in short, do what is needed to make the pull a success, it doesn't matter if your DPS drops for the duration of that pull, paying attention to meters is more likely to get the Tank killed than to be effective. Controlling in front of the Tank means that we can simultaneously watch your controlled target and the currently Tanked targets at the same time, this means we are not reliant on a startled cry when a Banish of Sheep breaks early (yes, even a hit capped Warlock or Mage can lose their Control target), and can react to this (typically it will move towards the healer). Furthermore announce the expected break time on a target, there are cases where you cannot maintain control, giving us two seconds to prepare is a lot worse for the group than letting us know ten seconds in advance because there might be a backup plan.
Now that we are on the same page as to what I mean lets discuss where crowd control takes place, and that is in front of, or to the side of the Tank. It might be really cool and effective for you to make the enemy run through the tank group, or pull it behind the Tank, but thats a horrible place, if something goes wrong the Healer is between the enemy we don't have threat on, and us. Turning around to deal with a target behind us opens up our back, and we lose all of our avoidance, this means our damage spikes hugely (typically moving from 40-70% damage taken, to 90-95% taken). If possible tank in the line of the pull, this means any target that will break has to move through the tank group, and additionally gives the Tank a known place they can move to to perform Area of Effect abilities (which break crowd control). As a Paladin I will Consecrate to hold Aggro from the enemies, this means a 16 yard circle on the ground is denied as a crowd control area, I can move backwards easily Consecrating as I go if needed to maintain crowd control, if enemies are behind me my movement is restricted.
There is one exception to this rule, and thats Kiting,often a target being kited needs a large area of space to be kited through, moving in a circle around the Tank is a typical path and acceptable provided the Kiter informs the tank when things look iffy such that the Healer can be repositioned away from the target. Ideally in the case of Kiting the Healer should move through the Kited enemy such that there is a larger gap between the Healer and the Kited target. This should ideally palce the Kited mob on the oppositte side of the circle from the Healer at all times, however due to the nature of the pull setup simply giving a little distance is fine.
Clearing up a well known fact:
Paladins are the multi-tanking class of choice, Consecrate will, on its own hold from a reasonable AoE DPS, or a low level direct targetted damage (so a Karazhan Warlock can likely nuke any target on a pull with minimal chance of pulling), however this is not to say that a Warrior or Druid cannot achieve the same feat. It is well known that a Druid can hit three targets with one of their main Aggro generation abilities, and a Warrior one, what this means in reality is that if you give a Druid or, especially, a Warrior a little time they can maintain sufficient threat on off-target enemies such that the Healer will not pull from them, and their Main target can be single target focused down with a little care (because they are spreading their Aggro generation). This is the same as a Paladin, you cannot expect to focus and go full out on a target we are not targetting in anything beyond the lowest level of content, give the Tank the time needed to secure the pull (unless you are confident you can handle any target you pull without assistance, because the Tank will likely let you keep it) and anyone can "AoE" or "CCless" Tank instances. This does mean that a Warrior can Tank Shattered Halls without a single Crowd Controlled target, you just need a good Tank that knows their class, and a group willing to give them the chance to prove it, you won't get it right off the bat as it does take effort but your Tank will be much better for allowing them to learn.
The Classes from a Tank's Perspective:
Now to the real issue that this topic was meant to address, what is it about classes that a Tank truly knows, focuses on, and expects you to be able to do. This list changes with the Tank's gear level and quite often this reduces the expectations on the classes from their full potential, however as I noted a Tank doesn't care how you do it, as long as you do it, use your abilities to their fullest or don't, but don't expect to skip the basics. The Tank will often assign tasks in a fairly generic way, such as "Sheep Moon", what this typically means is Crowd Control your target and don't let it kill you, or the Healer (since they will build threat on it due to the way Healing Aggro works), this doesn't specifically mean you need to Sheep your target, if you want tell the Tank that you can handle it and deal with it in another way, for example in Hellfire Ramparts the Shadow Casting targets can often be dealt with by a Warlock or Mage simply by attacking them, your soloing a target is both Crowd Control and damage dealing.