Dumbest things: redubbing Boba Fett, making Greedo fire, poorly done CG Jabba (I actually ike Jabba and wanted to see him, I admit the scene is marred by the "stepping on tail" cheap comedy and tossing in more pointless Boba Fett for the fanboys, plus the recycled dialouge, though the annoyance in Han's voice makes the repetition a little more tolerable), and last but not least, Hayden the ghost. Also, removing Vader's eyebrows seemed kind of pointless, as they gave him some character, plus the bald cap is even more obvious as his face doesn't feel weighted properly without his brows. And the various color screw ups on the official dvd's...
Stuff I disliked in the original originals are the garbage mattes, I hate those, and the badly blue screened stuff like the Rancor in ROTJ (love the Rancor, hate the effect that ruins the illusion). The inconsistencies in the TIE/X-Wing performance between "radar" and "window view" shots and the main shots was a little bit odd, and I think the SE improves upon that, but I guess I didn't really "hate" it before, only that it looks better now, one of the few non-purely cosmetic changes I approve of in the SE's. Or and replacing the cardboard extras with real people is fine by me.
And I disagree, CG Jabba circ 1997 looks nothing like ROTJ Jabba. He looks like a bug-eyed anime frog character. Some have pointed out that he's better animated than the 2004 version, but at least the 2004 version looks more like Jabba physically and less like a cartoon character. Closer, but still not quite there. They did a much better job making a CG Yoda, but he doesn't look quite like himself either. I thought TPM Yoda had a character all his own, even if he differed greatly from OT Yoda. Anyway, Jabba, they never got the eyes right, and that does a lot to define the character.