(Note: See Star Wars Re-View for further opinions.)
Surprising as it may be, I bought the Star Wars trilogies on DVD only in January. I've since been making my way through them, DVD extras and deleted scenes and all, and it has struck me upon finishing the prequels, especially upon viewing the deleted scenes, how just a little tweak in editing would make the last two episodes much, much better movies. (And given Lucas's penchant for going back and reediting his movies, it could not only even actually happen, but for once improve them! Knowing his CGI fetish, he could even invent scenes from whole cloth soon.)
First, beef up Padme. In both Episodes II and III, important scenes for Padme were cut that, I think, would have given her more motivation and interest. The deleted scenes in AotC are particularly important, because they're almost all about giving her some background and a more convincing POV in her budding romance. As I've said before, I've always thought her falling in love with Anakin was a bit undeveloped. Turns out, there was -- at least a little -- more explanation for her feelings; it just got tossed.
In Ep III, as released in theaters, all she basically does most of the movie is be pregnant and stand around her (fabulous, I do have to say) apartment wringing her hands. The deleted scenes actually gave her something to do besides spout silly lines about the lakes on Naboo, with at least the ghost of an independent subplot about the beginnings of the Rebel Alliance (with a young Mon Mothma who apparently took elocution lessions from Julie Andrews, Bai Ling wearing something that's actually about ten times less crazy than her real-life get-ups, two Lucas daughters, and more yummy, goateed Jimmy Smits screentime), which is better than nothing.
Though, in the end, I just never warmed to either Padme or Natalie Portman in the way I adore Leia and Carrie Fisher, Padme was kick-ass in the first two, which makes her passive weepiness in the last movie hard to take. I mean, I understand why Lucas didn't have a pregnant lady waddling around shooting at droid troopers, but he could have at least kept the stuff where she's still sitting around, but at least DOING something, in the movie. He so doesn't care about women. I mean, the Original Trilogy isn't overflowing with female characters, Leia and her awesomeness aside, and it's obvious that Padme, though again awesome, just wasn't any sort of priority. It's all about Anakin for Lucas, which is fine and all, but underserving Padme underserves the story.
Second, modify the ending of Revenge of the Sith, specifically the silly "Noooooooooo!" and the "She's lost the will to live!" cringe-inducing moments. Both could be easily fixed with just a little re-dubbing. Just have Vader silently, or with a strangled cry, wreck the operating room. And have that droid say something like, "The asphyxiation caused several embolisms/hemorrhages/strokes/whatever. We can't heal them fast enough." Those two things right there would have wrapped up the movie at least a little more palatably.
BTW, when I say the movies could be much better with relatively little work, I'm talking about Episodes II and III. I don't think any amount of polishing could turn the shit that was Episode I into anything other than a pile of poodoo. My Lord was that awful. Poor Jake Lloyd having to spout those TERRIBLE lines: that's child abuse on George Lucas' part that was. But at least he has an excuse in that he was an eight-year-old boy; Liam Neeson has NONE for the simply dreadful performance he sleepwalked his way through.
Some other observations:
I've also come to the astounding realization that Hayden Christensen isn't a TOTALLY horrible actor. He actually does some of the more comedic scenes, as well as action, rather well. It's all the angsty stuff that is just totally beyond him.
The writing, of course, is also terrible. I think the writing was just as bad in the originals, but Harrison Ford and Carrie Fisher had the panache and charisma -- or at least in the latter case the pills -- to pull it off in a way Hayden Christensen and Natalie Portman just don't, and Luke was a corny character anyway, so Mark Hamill was fine. Only Ewan MacGregor (whose steady transformation over the three movies into the reincarnation of Alec Guinness is really, really spooky) and Ian McDiarmid had the ability and the sense to work with camp that the material, I think, needed.
Of course, I think its also that George Lucas takes himself much more seriously and stultifyingly than he did originally. Thirty years of being in his own little kingdom of cinema (and growing jowls of Jabba-proportions) can do that to a man, and it weighs the prequels down with a portentousness that the originals, consciously patterned on cheesy adventure serials, don't.
One of the biggest mistakes of the prequels, I think, and one that editing unfortunately can't fix, is the constant cycling through secondary villains (Maul to Dooku -- which involved the criminal underuse of Christopher Lee -- to Grievous). They should have stuck with Maul or introduced Grievous in the second movie or scrapped Grievous to keep Dooku through the third. Palpatine is the main villain, of course, but he's in the shadows until the very end, and, the two Jedi battles aside, not an "action" villain. The secondary villain was supposed to provide the physical danger and the action. As it is, none of them ended up feeling like much of a threat. They just ended up being slightly-glorified redshirts who get in the way of the heroes' lightsabers.
Again as I've said before, making the Jedi total assholes takes away a lot of the sympathy and rootingness we were supposed to feel. They're just so damn smug and stupid. It's really amazing how truly, astoundingly awful Samuel Jackson was as Mace Windu. Though, to be fair, I think Windu was just a really awful character.
One of the greatest weaknesses of the prequels was the insistence on callbacks and fore/aftshadowing the original trilogy that gets in the way of telling the story of the prequels. Instead of letting the Rise and Fall of Anakin Skywalker, which is Lucas' "grand" story, unfold on its own, he insists on redoing the originals in broad strokes, with added Jar Jar. There's a difference between ironic resonance, fan service, and "Eh, I can't be bothered to come up with anything, so let's just redo that one scene from Empire with some CGI and different people."
And, actually, there's a lot of VERY interesting things happening in Episodes I-III with fathers and mothers, as well as misunderstanding and projection and denial, but I won't bore you, or maybe I'll just save them for yet another post sometime in the future.