My gripe is that ANH and ESB showed the rebels bickering with each other all the time while they acted like the Superfriends in ROTJ. It was like all the conflict between everybody got encased in carbonite and forgotten about. Say whatever you want about TPM but Qui-Gon butted heads at various times with the Jedi Council, Queen Amidala, Obi-Wan, Captain Panaka and probably other people even though they were all supposed to be on the same side.
Before the Ewoks adopted the rebels, they were badass. They were about ready to grill up smugglers with a side of Jedi at first. But after they adopted Han, Luke and Leia, they just became too cutesy.
The second Death Star... look, I'll acknowledge it serves a different purpose this time around. In ANH, it was the dragon to be slain. In ROTJ, it was the labyrinth, the hell/netherworld the hero must venture into to rescue somebody. But at the end of the day, they don't look all that differently or serve a much different purpose in either story.
Leia discovered that Adolf Hitler is her father and... what, just forgets it? If anybody found out Hitler was a great grandfather or something, they'd have some kind of reaction, even if it was destroy all the evidence and "silence" anybody who might know.
Harrison Ford basically slept through the movie.
Speaking of which, the stuff with Jabba and rescuing Han seems like padding. The rebels had to rescue Han but the ordeal feels a little dragged out.
Don't get me wrong, ROTJ has a lot going for it. The stuff with Vader, Luke and the Emperor is the dramatic high point of the entire saga as far as I'm concerned (but only with Shaw as Anakin's ghost) and the space battle hasn't been topped.