I like the instant feeback thing on the left - very slick!

My only complaint is that it forces you to sign up if you want to view the event details. I can do this with Facebook already.

I'd like to see more details in the "You're invited..." email. That way I could invite people like my parents (who vaguely understand email, but not how to sign up for this kind of service) to quick & easy events.

Forcing a sign up to see the event's message board, or something, might be a good compromise.

Site looks very nice! Very uniform design style - I don't love how all the headings are lower case, but that's probably a personal preference thing.