Big let downs...
Acting/Writing/Directing. I've been told that Hayden "wooden boy" Christensen actually can act, just not in any SW movie. I also found it hard to reconcile the facts in my brain, that Padme is the same actress as Matilda in Leon (which she played very well). Even if the story is badly told, it could have been much less painful with some decent directing of the actors, some of which are quite good in other movies >_
Worst performance...
Hayden Christensen. If the guy can act, then I hope he got paid well as he can't really use his role in the SW movies as a reference (if I were him, I might even consider a name change). He is just the tip of the iceberg though, as the only decent performance was by R2-D2.
Worst idea...
The move from story telling to techdemo for the newest CGI gadgets. Some kids might get a kick out of the battle scenes, but the story, which is not particularly good to start with, is so badly told it makes you cringe (Episode III was the first movie I've been to where people started leaving the cinema in droves before the end).
I hope Lucas hires story writers and directors for future projects