It's a lazy, stupid movie. All of the stuff on Tatooine in the beginning is pointless. Yes, it gets Han back so that he can stand around and do nothing for the rest of the film (except lazily be the solution to the love triangle) but that's it. It has no bearing on the rest of the film. Isn't Jabba's place filled with shady individuals? Wouldn't it make sense that maybe one of them knew something about a new Death Star? Couldn't that set up the rest of the movie instead of being just a pointless prologue?
Why does the deflector shield have to be dropped on the Death Star so that the shuttle can land on Endor? There's no shield around Endor. And if all it takes to have the Death Star lower the shield is a stolen ship piloted by rebels, then why isn't the entire rebel fleet five minutes behind the Tydirium preparing a sneak attack?
And once the rebels can land on the moon in their stolen shuttle, why don't they keep up the cover of supplying parts and a technical crew and land at that shield generator? Or maybe they could use that ship to blow up the shield generator.
The Emperor is a cackling buffoon. The galaxies biggest threat is....a Saturday morning cartoon character. Darth Vader is a subordinate to this clown? Somehow that doesn't add up based on the earlier two movies.
On top of all this, it's a really poorly made movie, with the pace of a dead snail. This is where Star Wars films started getting filled with superfluous material. Whereas Star Wars and Empire would cut into the middle of scenes, showing us the relevant information, Jedi starts the practice of characters slowly walking into rooms, wasting time before characters speak. It's a trend that would only get worse in the series.
Jedi ruined everything and foreshadows everything that was to come out of Lucas and the Star Wars series.
If you want to read more reasons why people hate this movie, go here.
Neil