Of course the acting (by the major characters) was bad. Lucas has found one Harrison Ford and a bunch of Mark Hammills. Yeah the plot was bad. The obvious way to plot Anni's turn to the Dark Side was Faust, selling his soul for immeasurable power, but that only comes into III as an afterthought, but Lucas wasn't capable of writing it and Hayden wasn't capable of acting it.
And Faust has been done so often, I'm sure that is why Lucas avoided it. Anni's turn started with the slaughter of his mom's killers in Episode II. It continued with his breaking of his training to have an affair with padme. Further along when he killed Count Dooku, which is something even Dooku would not have done.
After 9-11, my first thought was to nuke the bastards and if we had, we would have irreversibly gone over to the dark side. I'm glad for our compassionate, and christian president who had the morals and good sense not to do that.
Lucas did not want Faust. I think he wanted to show the difference between Christians bringing down the Roman empire and becoming the dominate religion for centuries and Hitler almost conquering the world. There were two very different paths to power.
I went to see it with my son this weekend (he'd seen it already, and humored Mom). The dialogue had some splinters sticking out of it, but I tried to ignore it.
The bald-faced attempts to slam W were just embarrassing, and met with stone silence in the theatre - no little "yeah"s or anything. Just dropped right into the punch bowl.
I did get a kick out of the little Rainman droids in the beginning who said "OW!" and "Uh Oh!"