So far I have a list of about 80 things, though this will fluctuate if I find more, or decide to ignore some of them.
Here's a selection of things I might change:
1. Replace "led by Luke Skywalker" in the opening crawl, because he's not really leading, is he?
2. There's a strangely linear feature in the atmosphere as Luke flies away from Hoth.
3. Align R2's text in the X-Wing cockpit so it doesn't look badly superimposed.
4. R2 looks TOO blue in some shots in space (as opposed to not looking blue at all in ANH).
5. Replace "You were lucky to get out of there" with "You're lucky you don't taste too good."
6. Make the Emperor scene closer to the OT/SE, and hide more of Palpatine's face.
7. Replace Boba Fett's voice. I wouldn't mind so much if the lines were actually delivered well, but they're not.
8. R2 uses the wrong tool to "access" the power outlet.
9. Something to liven up the painting of the "weather vane" inside Cloud City.
10. Extend the flashes from the lightsabres to illuminate the matte painting as well.
11. Replace Vader's "Inform my Star Destroyer..." with "Bring my shuttle."
12. There are two shots of Captain Needa saying "Yes M'Lord", one of which appears to have been shot on the wrong part of the set. The only fix I can think of is to duplicate the shot, which might make things worse.
13. The cables in Luke's artifical hand don't move in sync with his fingers.
14. Remove Clive Revill from the credits and add Ian McDiarmid.
15. Spell Denis Lawson correctly, again!
DE