On top of what Im sure everyone is saying that also fits my opinion, my biggest problem is that nothing felt like the same universe. Everyone felt like actors dicking around on blue-screens with no real human factor, and the actors seem to know that.
That and all their philosophies with the Jedi that I remembered from Empire Strikes Back as a kid, that really captured my imagination were replaced with stupid, uninteresting lightsaber fights, that never made sense to me. They just had to have the lightsabers out ALL THE TIME. To me, I always thought it made sense in the original trilogy when Luke would still fight with the same run-and-gun that Han would do, but he would pull out the lightsaber when shit got real. And it was those moments, that also made you excited, since you know someone was gonna get fucked up when the lightsaber emerged.
But it was just everywhere in the prequel trilogy. That and it completely changed Yoda when they had him go into his crazy stupid CGI lightsaber fight.
The entire time I just felt that Lucas didn't understand his own property- but then again he didn't have nearly as many people checking and countering his ideas, seemed like there wasn't anyone to tell him that his ideas were stupid, for fear that the 'glorious' George Lucas would not be happy with them- so they let his stupid ideas make it into the final film (see JarJar)