Adam Brody and the guy who played Chip were the only really good actors in it. Amanda Seyfried was trying, but, as I said, it was ridiculous that she was playing an ugly girl (I kept thinking of that scene from Not Another Teen Movie: "She has glasses! And a pony tail! She's hideous!") And Megan Fox is spectacularly bland and uncharismatic.
I also agree with the people saying the movie was trying too hard to be hip. Heathers is great, because even though they were making it with the intention of being totally different to all the John Hughes movies, they probably didn't think it was going to end up a cult classic like it did, and so it never feels forced.