Yoshi-P alluded to this a little himself in the recent Live Letter, mentioning that the greater complexity of the classes is making it more difficult to meet assumed DPS levels for raid content. It's possible that we'll see some streamlining of rotations and the like in 4.0 (they need to lighten keybind bloat as it is).

It also varies by class, too. I'm actually hoping to go back to SCH from NIN in part because I've been having difficulty maintaining my DPS and also adhering to fight mechanics lately, and I think I'll do better as a healer. We'll see, though.