As for twitch skills, I think there is decent amount of that in WoW and other MMOGs already, especially in PVP. If you gave the exact same characters to: a) a top 10 5v5 arena team, and b) a sub-1500 rated team, and had them fight it out, I guarantee the sub-1500 rated team would get stomped badly. The arena does require gear, but a lot of it is coordination, strategy, and some reflexive skill.
Regarding differentiation between characters in MMOGs besides gear - it would be extremely difficult, if not impossible, to ever see the amount of character differentiation like in DnD or other PnP RPGs. Mainly because in private D+D campaigns class balance does not matter, in MMOGs balance is everything. Also:
- if characters in the game are any significant time investment past something like Diablo 2, some form of respec is mandatory. Not to mention future buffs/nerfs by developers, which will happen, dictates a respec system is required.
- even in a classless system, certain builds will always be more effective.
- Since respec is must, the majority of people will gravitate towards the same set of builds
- an AA system where everything is available does not provide differentiation. It simply means max level = max level + required AA
So besides racials which are fixed, it will be very hard to make Warrior A and Warrior B in the same game significantly more different than they are currently in WoW, VG, or EQ2.