The main problem with the prequels is the way Lucas edits the films. He films a bunch of source footage, and assembles everything after the fact. He re-arranges characters, does artificial zooms, replaces dialogue, and what ends up happening is that the actors don't get to PERFORM. These are not bad actors, folks (well, except for Jake Lloyd) -- Lucas is diluting their performances by not giving them the context in which to act. That's also why many of the scenes seem awkward.
The prequel trilogy so far has not been as good as the original trilogy, but I still enjoyed Episodes 1 and 2. There are some really great moments in both movies. The last 45 minutes of AOTC are excellent, and I like all of Obi-Wan's scenes throughout the movie. For TPM, every scene with Darth Maul is terrific, and I really liked the pod race (two headed announcer notwithstanding). And the Emperor is my favorite character, so I ate up every scene with Palpatine/Sidious. Jar Jar I didn't really mind so much, because I understand what Lucas was trying to do with the character and with TPM overall. I'd say I like the Ewoks less than Jar Jar. But now Jar Jar's done his bit for the story, so I wouldn't be surprised if we see him even less in Episode III than we did in II.
As far as changes to the original trilogy go. When the Special Editions were released, there were only 2 changes I disapproved of: Greedo shooting first, and Luke screaming after he let himself fall in Cloud City. All the other changes were for the better, in my opinion. Better visual effects, making Cloud City seem more alive, the new ending of ROTJ which added much needed scope to the victory, giving a bit more background to Han's debt with Jabba, etc. Now Lucas is making some changes for the DVD's. Supposedly Luke's scream has been removed. Excellent. Ian Macdiarmid will be in the holographic communication with Vader in ESB, and the lines have been changed slightly. Excellent, in fact hearing the .mp3 gave me the chills. Hayden Christensen replacing Sebastian Shaw as Anakin's ghost -- evidently explained in Episode III, but either way I wouldn't say it BOTHERS me. Naboo (and possibly other cities) added to the celebration scene at the end of ROTJ -- why not? Doesn't change anything. Now if we can just get confirmation that Lucas has removed Greedo Shoots First...
To me, only those two changes (Greedo shoots first and Luke screams as he falls) actually changed the movies themselves. They changed the characters we knew, and THAT was wrong. But none of the other changes do that. All they do is add some flavor & life & eye candy to the movies. And if the new changes better tie the original trilogy to the old trilogy without altering the overall themes, story and characters, I see nothing wrong with it.
And remember, I saw the original Star Wars 14 times in 1977, and 12 times when it was re-released a year later. Star Wars was the most significant influence of my life throughout my teens, it was pretty much foremost in my thoughts most of the time. I was a hard-core fan. This is not to say I'm a blindly loyal Lucas-can-do-no-wrong fanboy. I hated THX-1198. I still don't really like the Ewoks all that much (and I *HATED* that stupid Nyub-nyub song that originally ended ROTJ). I wish Lucas would stop with the poop & fart jokes already. And I wish Lucas would let the actors act, and stop messing with every scene so much that their performances can't show through. But none of that was enough to make me hate the movies or the story or the mythology overall, or to write Lucas off as a "hack" or "asshat". Change is HIS prerogative. And if the new DVD's change the bad things back, and add new things that expand the story, that's a good thing.