I am an RotJ fan. On the whole, I like it and I think it's one of the few series ending movies that actually works for me. That is to say, I think it delivers on the main "promises" made by the preceding movies. The Luke/Vader scenes at the end are near perfect, in my opinion. The space battle at the end has yet to be matched or surpassed in terms of complexity and excitement.

HOWEVER (and I know there are whole threads dedicated to it, so I won't go into much detail):

The burping? The Tarzan yell?

The Ewok's cuteness factor and makretability seemed to be weighed higher than their believability or story relevance.

The aliens in Jabba's place seem to be "cuter" than their Cantina equivalents.

And most importantly, Han and Leia seem to be less three dimensional than their ESB selves. This can be blamed on the actors (Ford, primarily) but I think it was a sign of the story and style over characters problem that Lucas seemed to be developing.

I guess these are top of the list to me. Like I said, I actually like RotJ a lot. I have a hard time chosing between it and ANH for 2nd best of the trilogy. But I still think it bears the signs that Lucas was starting to make movies for his 3 year old daughter as opposed to his 18 year old self.