I've just finished watching the trailer for Star Wars Episode III: Revenge of the Sith for the twelfth time, and I'm still wiping drool from the floor. Lord, this is going to be a good movie. It probably goes without saying that I can't wait.
Looking at the few bits shown in the trailer, it appears that what drives Anakin's fall to the Dark Side is not just his fear and anger but also his desire for power. The trailer depicts Chancellor Palpatine telling Anakin that the Dark Side can give him abilities "that some would call unnatural", with Anakin responding "Can I learn this power?" Later, Anakin is visibly angered when the Jedi Council refuses to grant him the title "Master". (Which gives a nice spin to the line in A New Hope, when Darth Vader tells Ben Kenobi, "Now I am the Master.")
We can already look back to The Phantom Menace for traits in Anakin's character that lead him to the Dark Side -- chiefly, his pride in his being the only human who can steer a pod-racer. He's already a kid who wants power, who craves uncommon ability. And that's before he ever falls in love with Padme.
All of which goes to show that George Lucas knows what he's doing, at the basic story level. But of course, we can't give him credit for that, in this era in which "Dialogue is everything".
(BTW, a commenter in this thread at Kevin Drum's blog dredges up one of the most eye-rollingly stupid criticisms of the Star Wars prequels that continually arises: "Why is Lucas telling us this story, when we know how it ends!" What twaddle. Does anyone complain about Mel Gibson making a movie about Jesus, when we already know the end of the story? The fact that we knew that the boat sinks didn't seem to dissuade people from flocking to see Titanic. The idea that stories are only worth telling if we don't know the ending beforehand doesn't stand up to the tiniest bit of scrutiny.)