Having been immersed in D&D party creation lately, I was inspired to write my own taxonomy of the fourth edition party roles. This may be deathly boring to you, in which case, well, stop reading. But people seemed to enjoy my communication style on this sort of topic for WoW, and I like doing it, so there.
D&D 4e explicitly breaks character classes down into four roles: defender, leader, striker, and controller. However, when I think about how a class works, I just think about resources. Pretty much everything can be generalized as a resource. Spells? That's a resource. Hit points? Throw 'em on the resources pile. Potions, ammunition, special abilities, how many actions you can take in a turn, even how many warm bodies your side has - all just resources. Fights between the party and monsters are really just races to see who can deplete whose pile of resources fastest. When you think about how a character works, think of it in terms of how they (i) conserve the party's resources, and (ii) deplete the enemy's resources.
Defenders are characters whose special talent is making the bad guys use their resources inefficiently. They do this in a couple of ways.
Having good defenses, and marks. If your character has very good defenses, enemies who attack you are using their resources inefficiently. It's as simple as answering this question: if two party members are going to be attacked by an attack that deals 10 damage, and A has a 40% chance of being hit by that attack while B has a 75% chance of being hit, who would you rather throw under the bus? Now answer: who would the bad guy rather attack? Anybody can have a high defense. A defender is only making the enemy inefficient if she (i) has better defenses than the bad guys' alternative targets, and (ii) a way to incentivize the bad guys to attack her nevertheless. That way is called a "mark," and generally only defenders can mark the bad guys. When a bad guy is marked, he takes a -2 penalty to attack anybody other than the person who marked him. But note that merely being able to mark somebody is not enough. If you have high defenses, the enemy can sensibly not attack you. If you have marked the enemy and he wanted to attack you anyway, he's just doing what he wanted to do all along. But if you have high defenses and mark the enemy, you have degraded his efficiency no matter who he attacks. And the more inefficient the enemy, the more likely your side is to win the resources race.
Punishing the bad guys. A high-defense character who marks a bad guy is already making him use his actions inefficiently, giving the good guys a competitive edge in the resources race. Defenders also generally have a way to hurt the enemy if they attack somebody other than the character who marked them. For instance, if A marks bad guy X, and X still tries to attack good guy B, A will have some ability that interrupts X's turn and lets her deal damage to him. This is part of the Catch-22 that is how defenders contribute to the resource race, but note that now, instead of making the bad guys use resources inefficiently, you're directly depleting their resource pile by taking away hit points (and possibly a warm body or two).
Some defenders are better at having super defenses; others are better at punishing the bad guys, but I find that having a clear idea of which a character is best at helps me figure out how to best contribute to the team.
Leaders are characters whose special talent is in making their team use resources more efficiently. They do this in a couple of ways.
Healing. Restoring hit points to the team may seem like it's just restoring team resources, like pouring water into a bucket with a hole in the bottom. But that's only half the story. The other point of healing is that dead characters take team resources with them when they die. The sorceror who died with a devastating spell uncast, the knight whose plate armor is now lying useless on the floor - all those things are team resources. When a leader heals a party member, she should think not just about who is most likely to die but about who would take the most valuable resources with them if they did. If Johnny the wizard has used all his spells but Alice still has a special attack she's yet to use, all things being equal, Alice gets the heals.
Moving the party. Turns are resources just like everything else. If you have to spend a whole turn moving into position before you can unleash your devastating attack, you're being less efficient than if you could unleash that devastating attack right now. Some leaders are particularly good at moving party members even when it isn't their turn, which can ameliorate this problem. But note: only if doing so is more efficient than what the leader could do herself. Suppose A is a leader who fights with a bow. If she can deal 10 damage to monster X right now, or move B into position to stab X in the back with B's daggers, what should A do? Or suppose X is threatening B, and A can move B out of the danger zone - or deal 10 damage with her bow. What should she do? There's not necessarily a correct way to assign mathematical weights to this question (although sometimes there is, with the simpler versions of this question). The important thing is that A is thinking not just about her ability to move the party into attack position, or out of danger, but also about what resources she is consuming (i.e., her own turn) to do so.
Helping the party attack. The third thing leaders have in their bag of tricks is the ability to help the party attack directly. In some cases this literally means an ability for A to give up her attack so that B can attack instead - obviously only worthwhile if B's attack depletes more resources than A's. In other cases it's a bonus to the team's attacks - for instance, A hits X, and for the next turn, everybody else who attacks X is more likely to hit, or does more damage if they do hit. There is a double efficiency here: by making the party's attacks more likely to land or hit harder when they do, the party's actions are used more efficiently, and of course a successful attack depletes enemy resources as well.
As with all roles, some leaders are better at one trick than others. All of them, though, pretty much contribute through some combination of these three.
Strikers are characters whose special talent is in depleting the enemy's resources. There is really only one way to do this, and that is to attack the bad guys; to assist in this role, all strikers will have some way to increase the amount of damage they do, make their attacks more likely to hit, or both. The more a striker can maximize how hard she hits, maximize her chance to hit, and minimize the time she needs to get into position to hit, the more efficient she will be.
Solving that puzzle is only part of what a striker does, though. Consider this question: monster X has 100 hit points left, while monster Y has only 10. Striker A can attack either of them for 10 damage. Who should A attack? If you answered, "Well, how likely is A to hit each of them?" good for you. You have grasped a nuance in how A can best use the team resource that is her turn. Also note: if X is hit, he will continue to deplete party resources (i.e., attack the good guys); if Y is hit, he will not continue to deplete party resources (i.e., he will be dead). That certainly weights things in favor of hitting Y. A related wrinkle that a striker will often have to decide is this: if I can't kill X or Y, which is more likely to take away more party resources on its turn (i.e., if neither can die this turn, who should die first)?
These are problems that all characters should consider, but as strikers have the most direct role in taking away the bad guys' resources, it's particularly important for them.
Controllers are characters who specialize in making the enemy inefficient. They share this trait with defenders, but they inflict inefficiency in very different ways:
Area of effect attacks. Attacks with an area of effect (often abbreviated "AOEs") can damage many enemies, but generally only kill very weak ones. Killing the weak may seem more like a matter of taking away the enemy's resources than degrading their efficiency - and you can think of it that way. However, the really weak enemies ("minions" is the technical term), pound for pound, do more damage than other type. Consequently, pushing them off the table with a broom actually takes away the bad guys' most efficient killers.
Moving the enemy. Moving party members into attack position, or out of danger, makes the good guys more efficient. By the same token, moving the bad guys into danger, or out of attack position, makes the enemy less efficient - and for that matter, can make your team members more efficient, as well. Some controllers do this by making it hard or impossible for the enemy to move in certain places - erecting an invisible wall of force, say. Other controllers do it by literally picking the enemy up and moving them.
Hindering enemy attacks. If the party is more likely to hit, its attacks are more efficient. Similarly, if the enemy is less likely to hit, its attacks are less efficient. Controllers have a variety of ways to hinder enemy attacks. They can blind or stun the enemy, for instance, but they could also cause the enemy to take damage when they move next to a party member, or perhaps immobilize the enemy so they can't even get into attack position.
Again, not all classes are equally good at all the signature tricks of their role. The point of thinking about things this way is that pretty much all classes' primary abilities fall into one of these categories. Knowing what they are is a useful way to keep straight in your head what you should be doing in battle. But it's also a way to keep your character organized in your head. Are you a defender? Which of the two defender tricks does this ability tend towards?
Well, that's all. Hope this was useful to you, or at least fun to read.