What I would have done:

Replaced the Death Star II with an Eclipse-class Star Destroyer, and have it serve as more of a setting rather than a MacGuffin.

Cast a young, attractive actress to play Palpatine. Clive Revill would have been retained as the voice of Palpatine.

I would have had Palpatine's little cadre in the beginning of the film consist of Sith Lords/Dark Jedi instead of funny looking men in goofy headgear. This would show that Vader isn't the only high-ranking darksider in service to the Emperor, and it would help to set up a sequel trilogy.

I still would have kept the "rescue-Han-from-Jabba's-palace" subplot, but reworked it heavily to make it work better (I don't know precisely how I would go about doing this, however).

Placed Jabba's palace on another, new, world rather than on Tatooine.

Omitted the stupid "from a certain point-of-view" speech. Ben will admit he lied, but that he did it only in Luke's best interests.

Omitted the bit with Leia as Luke's sister/Vader's daughter.

I would have shown Sullust.

Changed the Ewoks to Trandoshans, Bossk's race (hey, I'm trying to be original, here!)

Changed the whole method of how Palpatine & Vader go about seducing Luke to the darkside to make it more believeable. The whole "if-you-strike-me-down-in-hatred-you'll-join-me" bit is just dumb and smacks of future PT developments.

Had Han die heroically on Endor.