You are wasting the precious few seconds of first impression you get asking them to sign up for something they don't even know if they will like yet.

Bring them straight into the flow of your site, and let them signup only when they want to, not when you want to.

You can whet their appetite by showing other people interacting, no need for them to interact themselves.

Take YC for example, I bet there are hundreds of people who just lurk and read submissions/comments, but if they really wanted to reply to one of them, then they sign up.