Firstly, he's not a good director, not very communicative...at least not in a way that actor's seem to respond to. Just listen to the way he directs the effects guys in the behind the scenes docs, and when they show it, the actors. He doesn't seem to give them much to go with. He directs pretty much the same way that he delivers a commentary on the dvd. He had a stellar cast for these films, yet his directorial skills were such that he could not coax convincing performances out of them because he's not good at conveying what a scene should encapsulate from an emotional perspective. Plus, look at the diaolugue he was giving them to work with. Ewan was probably the most ethusiastic of everyone involved to be a part of this. By the time it was all over he was probably ready to fire his agent for getting him into this. And, well...then there's Liam Neeson. Another guy who was excited to be a part of the films and lived to regret it. The dude flat out refused to do the voice over for the big explanation sequence at the end of ROTS that forced Lucas to mention it only as a cheap tag-on that almost seems like an after thought at the end of the movie.
Secondly, as Lucas has proudly stated, he doesn't shoot the movie, he shoots "around the movie". He shoots the movie almost as if he were cutting pictures out of a magazine to make a collage. He winds up with miles of "clips" that he then tries to assemble into a movie. Since he essentially makes the movie in editorial, scenes often become disembodied from where they should have been in the movie. Things get moved all around not just within a scene, but throughout the entire movie, and you wind up with awkward cuts and beats that don't seem to flow together because they were never really meant to fit together. There's no emotional core or rythm to it. It's like trying to build a song out of a single note. His famous line is, "we'll take care of that later", meaning in post-production. That's his answer for the whole movie. Sometimes doing things differently is called genius. More often than not, though, it's called stupid.
Lastly, I know they're tech films, but way too much green and blue screen shooting. You have to give these poor folks something to work with that allows them to believe that they're a part of this world. You can't just show them a sketch and say, "this is what it'll look like". All day in a green or blue room is one thing, monotonously boring, no matter how good the actors. Once they lose interest in what they're doing, or can't "feel it", that can't be fixed in post. You can't CG performances George. You have to get those the first time around. And if you don't know how to get them, then you hire someone who does.