1) giving alien races cultural accents rather than language. Having Threepio translate Artoo, and Han repeat back to Chewie everything he said was genius. It worked so well. Because there's so few subtitles, you don't notice as much, but much of the original trilogy was communicated without English. Both new movies fail here, it's either cultural accents (mexican for the junk shop dealer, vaudeville for the announcer, Japanese for the trade federation) or subtitles, and the universe isn't as good.
2) contrived settings. Why the crap is Amidala holding an audience in full dress on her transport ship, to "commend" a droid? Doesn't make sense. And so forth. Doesn't feel natural.
3) obviously the script was written in "higher language." Anakin and Padme don't use contractions very much, for example. However, they still behave and act like normal people you'd see in a TV mini-series. This disconnect makes the whole thing feel unnatural and forced. If you don't have Sir Alec Guinness, you can't expect a shakespearean performance so don't write shakespearean.
4) the movies rush along to endpoints. Lucas is trying to get from Old Republic to the start of A New Hope. He has certain conditions to be met, and the movie seems to be driven by navigating from condition to condition (war, empire, anakin to vader, marraige so that luke and leia can be born...) rather than unfolding naturally. It's a tough situation to be in, and it seems to be too much for Lucas. Again, they feel unnatural.
That being said, as I get used to them, the movies get better. I liked AOTC just because it was exciting. Going back to TPM, I realized there were just a few sticking points ("ye gods" - jarjar (hello? gods? in this universe? it hurt me., midichlorians, "there was no father", "are you an angel?", the attack planning scene with stilted dialogue) and the rest of the movie plugs along pretty well. Like my four points above, AOTC suffers from most of the same problems. A few less major sticking points, but still forced. In the end, it was a fun romp with an exciting battle and tie-together, with a good lead in to the next movie, so I left happy.
My first realization was that because of the Star Wars name, I had expectations which led me to dislike the movies. Later I also realized that without the Star Wars name, I would have watched them once, enjoyed them, and never thought of them again.
Now the clone wars cartoon series; that was good star wars. Any input...is it just because it's 5 minute serials and cartoons that it felt so much better, or did it just better capture what star wars is supposed to be?