James is the only vampire we meet who isn't beautiful. Why?

Beauty is in the eye of the beholder. Perhaps he wasn't beautiful by Bella's standards..... or beacuse he was trying to kill her and she didn't have enought time to ogle at his beauty ;)

Because before he was a vampire he was ugly. Once you become a vampire, your looks are enhanced, so if you're average-looking as a human, you'll be a beautiful vampire. If you're an ugly-ish human, you'll be an average vampire.
I just read all that on the Twilight wiki, because I was wondering the same thing. I honestly don't believe in any of it.

Maybe because she wanted to mark the differences between the Cullens and the bad vamps by making him ugly.
Plus, Exceptionally stunning humans make extraordinary vampires. So maybe he was just not that great looking as a human

Maybe it's because of the author's inlcination towards the Cullens(the good guys).

I never noticed that before. Perhaps because he's the first villain? Maybe Meyer originally wanted to mark out her villains by making her plainer looking than the heroes, and later changed her mind but didn't alter James' looks? The Volturi are also described as less attractive, with creepy-sounding paperlike skin.