Playing every single class constantly GCD locked, without even a solid two seconds of breathing room, sounds pretty hellish for a tab-target game. Sure, you've got games that go full-ARPG with their combat like TERA, Neverwinter, SWTOR, and WildStar, but how many players do those games have, again? I think people grossly overestimate how well full-action gameplay works with MMORPGs, especially when you concentrate a shitload of players in a small area, and I certainly don't trust the current crop of devs to pull it off with the necessary skill to turn it into anything less than a total shitstorm--they can hardly manage the gameplay they've got now.