This is why the GCD will never, ever be removed.

They have stated several times that the GCD is also to keep the servers from being nailed by thousands and thousands of commands at once. Everyone in a raid mashing things as fast as they can would be bad. The entire server doing it would slow everyone down.

The counterpoint to that is that Diablo 3 is basically a single player game using an MMO backbone; you press a key in D3, and that gets sent to the server, and then the server sends that back to you. Thus, it IS possible to have an online game that has no GCD on abilities.

Will it work in WoW? Maybe, maybe not. It would be interesting to see if Blizzard experiments with a total removal of GCD in the MoP beta, though.

Diablo 3 supports up to 4 players in a game together. WoW handles 100s in the same place together, often fighting each other. D3 has way less data transmission and processing overhead than WoW does.

This is why MMOs employ a GCD.

Except that even in Diablo, there is a cooldown. Granted, it's tied to the animation, but there is a period of time where you can't use another ability.

I remember in Diablo II, there was even an issue where the block animation prevented you from doing anything, therein causing you to cc yourself if your block chance got too high