I've seen EQ mentioned a few times in this thread and I have to say the one fundamental thing that WoW lacks in raids that EQ had as far as raids in rolls was a debuff role. Originally Warlocks were said to be the "debuff class." When I say debuff I don't mean a single curse or something I'm talking about the role of a Shamen and Enchanters in EQ. Tash, Mala, Slow etc... Somehow between Beta and now the line between debuffer and high DPS got obliterated and this game is too far along to change a current class to this role, its going to have to come from one of the Hero classes.
Problem is balancing such role in small scale PvE, raids, solo and PvP. Additionally, you got to make debuffing fun
. I mean, think about that - after the fight, people are checking their DPS meters. What can our debuffing warlock boast about? "Hey, I was channeling Slow for 80% of the fight and used Weakness twice when boss were about to use his special, yay!" Then after 10 raids warlock is spent and rerolls. It would be something along the lines of healing, but significantly less challenging.
Additionally, gear-focus of the game would make debuffing class unable to progress. You reach lev. 70, you get your max. lev. spells... then what? Gear only makes your DPS, healing OR mitigation better - but there is no gear that makes you buffs or debuffs more powerfull. Basically, fresh lev. 70 pally got just as powerful buffs as pally-the-Illidan-killer. It would be the same with debuffer class.
For debuffer and buffer classes work game would require completly different reward and character development system, that actually rewards acts of buffing and debuffing alone, also making them much more engaging then right now. People like to click buttons, preferably different buttons in different combinations that depend on situation around.