I've never had any problem with ANY of the Star Wars actors. The problem has always been with the writing, which was only done well in the first three movies before they had any idea how successful it was going to become.
I don't think that they even got cocky about it, I think it was just the pressure of living up to expectations. At the end of the day, the main problem with the prequels was that it just tried to do too much, introducing new characters but not really having time to flesh them out, elaborating on back stories but having to rush through the main points, having to put in a political theory lesson into it but not taking the time to explain why we should care about that. The original trilogy didn't have that problem because they weren't expecting it to do as well as it did so they felt free to keep it simple.