Here's some that standout in my mind:
A NEW HOPE
STORMTROOPERS ON DEWBACKS: Totally unnecessary add on. Utterly distracting to suddenly have 1997 CGI "Jurassic Park" type creatures inserted into a 1977 film. If Lucas hated the scene so much, he could have just trimmed the opening of it showing the trooper ontop of the immobile dewback and kept the rest the same.
BEN KENOBI’S HOME: Obi-Wan's home in the original film was perfect for a poor hermit. Now, suddenly he has a fancy hideway (bigger than the Lars homestead) with a sattelite dish in the front yard.
ARRIVAL AT MOS EISLEY: Sure some of the changes are definitely improvements and show the vastness of the spaceport. And how crowded it is. And removing the pink blob under Luke's speeder was nice. But Lucas had to go overboard and insert these unfunny slapstick moments with Jawas, rontos and dewbacks. All of these distractions take away from the performances of the main characters. And Lucas's insistence and including loud, obstrusive dewbacks into every scene overshadow Obi-Wan's classic mind trick to get passed the stormtroopers and 3PO and R2's banter outside the cantina. Not to mention, all of the CGI technology used makes the rest of the creatures in the cantina look dated.
GREEDO SHOOTS FIRST: Without a doubt, the worst change to the original films. Simply because it looks completely awkward. And how could Greedo miss Han from that close range? Was he trained how to aim by battle droids? In 2004, Lucas changed it again so that Han & Greedo shoot at pretty much the same time. Still sucks.
JABBA THE HUTT INTRODUCTION: Always sounded like a cool deleted scene until we actually see it as part of the final film. The dialogue is repetitive, nothing new here that we didn't learn during the Greedo encounter. Jabba looks nothing like he does in Episode VI. Plus he's skinny and mobile, everything he's not in "Jedi." And Han stepping on Jabba's tail and calling him a "wonderful human being" makes the scene a train wreck disaster that doesn't belong in AFI's best films of all time.
THE EMPIRE STRIKES BACK
NOTE: Changing anything in the classic film should have been considered blasphemy. Like damaging the Holy Grail. Empire was perfect the way it was, from 1980 to 1997, before Lucas had to toy with it.
THE CLOUD CITY LANDSCAPE: Sure the shots of an expanded Cloud City are interesting to look at. But they look like something out of a totally different movie. Particularly when Lando tells everyone to evacuate the city. The footage seems way too technologically advanced compared to the late 70's effects of the original.
VADER'S ARRIVAL ON HIS STAR DESTROYER: I don't know why Lucas felt we needed to see Vader arriving on his Star Destroyer after leaving Bespin. In Jedi, the Emperor tells Vader to return to his command ship. Next scene, Vader's on his command ship. We don't need to see arrivals & departures inserted into every scene. The audience can follow the story just fine without them. And instead of the cool line, "Bring me my shuttle," we get "Alert my Star Destroyer, to prepare for my arrival." Then Vader's shuttle is shown slowly arriving on his ship. Except it isn't a Star Destroyer, it's the Death Star from "Return of the Jedi." Lucas inserted this lame outtake from the beginning of Ep VI instead of taking the time to film new footage. Weak.
BOBA FETT'S NEW VOICE: Jason Wingreen's original dialogue as Boba Fett was perfect. Gave the bounty hunter a certain sinister quality. Temeura Morrison (who played Boba's father in Ep II), doesn't have the same evil delivery. Boba's not as mysterious now. Changing Boba's voice is just another desperate attempt by Lucas to tie in the classic films to some of the wackier plot points in the prequels.
CHANGING R2'S DIALOGUE: After R2 escapes the swamp monster on Dagobah Luke used to say "You're lucky you don't taste very good." Now he says, "You were lucky to get out of there." Like changing OT dialogue to reflect the PT. The original line was funny and clever. The "lucky" line isn't memorable at all.
LUKE'S SCREAM: In 97 Lucas inserted a loud scream from Luke when he falls to escape Vader. It was actually the Emperor's death scream from ROTJ. Thankfully, Lucas removed this awful change for the '04 DVD release. It was probably an oversight by Lucas. I'm sure he meant to keep it in just to annoy the fans.
RETURN OF THE JEDI
NOTE: Much about "Jedi" should be changed. If there was a way to make Jabba more realistic (and less static puppet), re-insert the sandstorm sequence, digitally replace the Ewoks with Wookies (as Lucas originally envisioned), and re-edit the action sequences so they're not so unbelievable, I'd say go for it. But instead Lucas has made many of the bad scenes even worse...
THE DANCE NUMBER: The cantina song in "A New Hope" was nice because it didn't distract from the plot. It was a nice, harmless instrumental backdrop to the scene. The song in Ep IV is just in your face. And totally disrupts the film. The original song in Jedi, "Lapta Nek" was bad enough. Now, Lucas includes an even lousier song, "Jedi Rocks" and some annoying CGI muppets. The scene looks totally unbelievable now. With a bunch of static, immobile puppets now surrounded by fully mobile CGI characters. It's amazing Lucas would pick this as the one scene in Jedi to "improve."
THE CELEBRATION: I'm glad Lucas dropped that godawful "Yub Yub" song in place of a more dignified John Williams musical number. And it was kind of cool to see Coruscant, Cloud City and Tatooine celebrating after the fall of the Empire. Then Lucas had to go and include Naboo, which for some reason, was the most important planet in the universe during the prequels. And of course, Lucas had to put some Gungans in there, yelling and hollering "Wessa free!" Who knows, maybe Jar Jar's in there somewhere. I knew Lucas would find a way to sneak him into the classic trilogy.
THE GHOST OF YOUNG ANAKIN: Instead of the classic version of Jedi, where we see Sebastian Shaw standing next to Obi-Wan and Yoda, we get a creepy looking Hayden Christenson looking totally out of place as a "force ghost." The scene was fine the way it was, and made a lot more sense. I could have dealt with the change if they would have aged Hayden 23 years to make him look he would at the time of his death. But making him 25 years old is a really big stretch, even in a fantasy film. Plus, he looks nothing like Sebastian Shaw.
CHANGES IN HAN SOLO'S DIALOGUE: When he's rescuing Lando from the Sarlaac monster, Han used to say "It's alright. Trust me." Now he says. "It's alright. I can see a lot better now." Again, the line worked better before. Of course, Han's signature line of "Trust Me" is sort of tainted by Anakin's misuse of it in Episode II.
All in all, I think the original 3 films were fine the way they were, story and character wise. Sure some of the technical effects could have been improved and some deleted scenes re-inserted. I wouldn't have blamed Lucas for cleaning up the effects for the special edition release. But Lucas went too far. Drastically altering the tone of some scenes and making some of the CGI additions stick out like a sore thumb. It's a shame we can't get the "best of both worlds." A version of the original trilogy on DVD that has all the improved sound/visual effects, but none of the over the top changes