Lucas’ whimsy and energy (creative not physical, he barely moved the camera) worked in a team of people where he had to fight to keep his ideas and notions in the mix. Once he became the Emperor and anyone that might obstruct him was removed he had nothing to steer him on course.
Not really a valid point, since he was the “emperor” ever since ANH was finished. Even in ANH he had mostly free hands since studio didn’t seem to be particularly interested in the film. Of course he couldn’t directly fire people in ANH but he could in ESB and ROTJ. The way I see it, he had more sense and was a bit more open to outside input during OT. Still, judging from screenplay meetings transcripts from ESB and ROTJ, he pretty much rejected any suggestion he did not like. Perhaps in PT his team wasn’t giving him any suggestion at all.
He was emperor for ESB but he had a director who could stand up to him. Not the case in ROTJ, and you can see the prequel mentality creeping into ROTJ if you are paying attention. Burping alien outside Jabba’s palace, anyone?