The main problem for me is the tone of the piece (it feels like the film makers are in a rush to get this Star Wars thing done and dusted, which to some extent they were).
The aliens in Jabba's Palace were a bit too cartoonish, admittedly some of the aliens in the Cantina sequence of ANH were too but that could be put down to lower budget and lower expectations after ESB we had come to expect a more polished and thought out series of films to continue on and instead it became a rather goofy love affair with Jim Henson because Yoda had been such a hit, which pathed the way for Jar-Jar decades later).
The Empire seems very big in ANH and ESB but shrinks in scale in ROTJ because of the percieved need to wrap it all up in this one film (which is compounded by the removal of all the characters which could pose the dreaded "What happens next?" question (Jabba, Fett, Yoda, Vader, even The Empire itself all fold in one film when each should be an event or needn't happen at all).
Obviously Han needs to be rescued so Act One is logical (but should be played less for laughs), Jabba's palace should be a place where danger is real and possibly we could have lost a major hero here (Lando or Chewie seem to make sense) this would make the beginning of this episode as tense as the ending of ESB.
Fett should either not be in the film or be doing something else and doesn't need to die.
Yoda dying of old age at that point where Luke still needs training doesn't work either.
Vader needs to be more active a threat, so possibly keeping the Emperor out of the film or slowly introducing more of him would have given him more room to do what he does best and having a rival for him (in the form of Jejerrod) to put down was a good move in the early treatments which they were unwise to remove.
Rather than going against a new Death Star they should have had a production line, a giant space dock churning out lots of the things.
The destruction of the Death Star in ANH was a real feel good moment in that film and a major victory for the Rebels, if the Empire started to mass produce the things the Rebellion wouldn't stand a chance so having a Guns Of Navarone story where this is a now or never situation for the Rebels would be bigger than destroying just one more Death Star.
Lucas explanation for replacing the Wookees with Ewoks never made sense.
Chewie is a co-pilot but that doesn't necessarily mean that all Wookees are technologically advanced.
He could have been trained to fly the Falcon just as a person from an Amazonian Tribal Society could be trained to fly a plane but the rest of his tribe still live a primitive lifestyle.
The idea of the banishment of Jar-Jar could be a re-cycle of what was originally intended for Chewie.
He was banished and had to learn to use technology to get along in the sort of world Han moved in and returning home for him may have been made difficult for him but necessary for the plot.
The battles in space and on the ground should feel like real battles with a cast of characters we know a bit about so when they die or are threatened we care.
In a few moments of screen time Red Leader and Biggs come across as real people painted in broad strokes, I actually cared when Porkins blew up.
I didn't really care at all for any of the pilots in ROTJ, even Wedge seemed to be protected by a force field of "they wouldn't dare" energy.
With my suggestions over in the ESB/ROTJ Wishlist I've tried to promote some ideas to put back in some of that missing scale but I think the real problem was in the script stage.
They took away a lot of the wrong things from earlier draughts (some of the changes were very necessary, Ben coming back to life for example was a bit out there) but Lucas does seem to get a bit twitchy about certain ideas and his heart didn't seem to be in it so much as fear of spending too much and not making enough back.