Let's see, negative changes from Episode IV:
-They ruined the Krayt Dragon roar that Obi-Wan uses.
-Mos Eisley is now populated by retarded looking dinosaurs.
-Replacement of the wolf man with the wierd reptilian thing (the wolf man, though cheaper, looked better)
-Greedo still shoots
-Gratuitous Boba Fett shot at the end of the Jabba scene.
Advantages would be the nicer color quality, with Luke's training period excluded, the lightsabres, and the Biggs scene.
On the ships, I'm sort of ambivalent. They look better, but in some scenes they just seem a bit off.