I'll add one more issue with MTurk: binary decisions. Some of my tasks can't easily be broken down into accept/reject, or sometimes a worker gets something close but not all the way there. I want to be able to say "hey, you did pretty well, but please make these small changes and we'll accept it."
But that's not possible right now. You get the HIT and it's accepted or rejected. The result is that we have to sometimes reject workers who are making an honest effort but didn't sufficiently complete the task or we have to accept sub-par answers so as to not have to deal with worker blowback.