Baby Samurai said:
To be honest, the terms defender, striker, controller and leader as roles are all new to me and I've been DMing/playing for 20 years.
It wasn't until my older brother started playing WoW that I heard the term "tank" for a meat-stick.
So I think unless you are familiar with MMORPGs, there's a chance you won't be familiar with any of these terms (like me).
The terms defender/controller/striker/leader aren't familiar to people who play MMORPGs either. WotC just made them up. Some of them correspond to terms MMOers use, but only in somewhat inaccurate and misleading ways.
Defenders are the traditional D&D meatshields. The guys with lots of armor and lots of hitpoints who fight at the front of the party, who the wizard tries to hide behind. They also get to do a lot of damage with their weapons while they're at it. (This distinguishes them from the corresponding MMO classes, who usually suck at dealing damage and rely on "taunt" abilities to make enemies target them. Mearls has explicitly stated that D&D4 defenders will
not be like that. Thank gods.)
Leaders are an expansion of the healer role D&D has always had. They heal, but they also buff other PCs' stats (e.g., bard music, spells like
bless) and grant extra actions and the like (think White Raven maneuvers from Bo9S). The name "leader" is probably just an attempt to make a support role sound cooler. ^_^
Controllers do what D&D players have taken to calling "battlefield control". Stuff like
wall of spells,
web, black tentacles, and other things that shape the surrounding area to the PCs' advantage. Area-effect damage is also part of the controller shtick, because you can use the threat of it to control how the enemies arrange themselves. Summoning might also be considered a controller thing, particularly if you have some choice over where your summoned dudes appear.
Strikers are about dealing damage, but we already know that controllers cover most of the area-effect damage and that defenders (or at least some defenders) are top-notch damage dealers, so what's left for strikers? They haven't told us clearly, but my impression is that strikers are about selecting key targets and dealing lots of damage to them in hopes of finishing them off quickly. For instance, a rogue who uses Tumble to get past the enemy's melee guys and hit their vulnerable spellcaster is acting as a striker. Even if they don't deal more damage than defenders, strikers can make a role for themselves by applying that damage where it does the party the most good.
The point I'm trying to make is that all of these roles have existed in D&D for some time now. All WotC is doing now is identifying them, naming them, and designing their classes to make sure that every character can adequately fill some role and thereby contribute to the party. They're not copying MMOs, they're just trying to avoid situations like the 3e monk, who isn't very good at doing anything except maybe staying alive.