Re: Episode III
Well, in Episode 1 you've got pretty much every exterior shot on Coruscant which makes it look like a planet made up of borg cubes with the occasional copy/pasted tall building, the battle scene with the Gungans at the end, the umpteen CG speaking characters that look like cartoons rather than actual species...
In Episode 2 there's the tour through the cloning facility, a lot of the bug arena shots, the sand dune Padme lands on, frolicking in the prairie on the CG pig-rhinos or whatever they are, the robot factory, the asteroid scene with Jango Fett shooting at Obi-wan...
In Episode 3 many of the shots on Grievous' ship - especially in the corridors, the shot where Obiwan jump down to confront Grievous and is surrounded by CG, the fight over the lava (the part when they're both on those little floating droid things especially), the "NOOOOOOO!" scene where Vader crushes shit with his brain...
In all of them there's a lack of interaction between anything CG and anything real. There are only a few sequences where the live action characters ever touch or grab or are touched or grabbed by anything that's CG. The light saber fights mainly just involve the jedi swinging their glow sticks around at random and having animators put droids or lasers in there for them to swing at. They rarely ever look at what they're attacking. The movies are just a really expensive version of those Star Wars Kid effects videos where people would have him deflecting blaster shots and killing little droids. The first movie has some insane bloom going on around all the windows.
There are good things, tho. In all of the movies there are a lot of good Tatooine shots, traveling through the planet core with the giant fish things looked really good, all of the evening and night shots and pretty much all of the space shots look great and the later shots of Coruscant look better than they did in the first movie. I'd say that the best shots in all three movies are the ones that are the darkest with interesting shadowplay going on. Most of the time, tho, those are shot on a complete or mostly complete set and the mattes just consist of lights in the background that get blurred.
I think the thing that throws me off the most throughout the entire series is how sterile the environments are. There are shots on Coruscant in Episode One where you see people walking around on platforms going places, only the scale of everything is so massive that they'd have to walk several miles over flat, perfectly pristine, lifeless pavement before they ever got anywhere. The lack of even potted plants is kindof crazy. How do they generate oxygen? And the planet is littered with giant, mildly stylized statues of naked, sexless people that all seem to have been created by the same guy.
I mean, they mostly look good I guess. I can't really complain about how well that stuff is rendered, but it just looks fake and wrong. The environments are all so vast in scale and yet so sparsely populated, and so well manicured. There are cracks and some wear here and there, but it's just so unsettling and bizarre that it doesn't matter how well it's rendered. It will always look fake.
Compare it to the shots of Rome in Gladiator. Huge scale, but loaded with people of all shapes and sizes and dress doing all sorts of things. A lot of the shots there are just as fake as anything in Episode One and it was made around the same time, but feels a lot more real. You look at the exterior shots on Naboo in Episode One and it kinda feels like they forgot to add characters to them.
And the animation on a lot of the CG characters kills the effect as well.
I dunno. Not much more to add, really. I, myself, couldn't have watched those movies much even if they had been decent or good. There's too much stuff in them that visually doesn't work for me. It's like the Rancor in Jedi. That thing has never worked for me, but it doesn't matter cause I only have to sit through it for that one scene. I can let it slide, even tho I don't fully buy into it. With the prequels it's like that for the entire movie and I can't let it slide.