I debated this for so long. My final rational was that I was going to lose some people but Twitter helps people connect. Since the site is partially about connecting you with others, I decided to give it a go. If it blows up, I figured I could offer email-based registration on top of it.

I'd suggest doing at least email-based registration. Email is still the most common form of communication online. Although a lot of people have Twitter accounts, there aren't as many power users as email.

It made me go get a password reminder for Twitter, dammit. Probably a good thing, all in all, but still. Good for a beta, but add Facebook and email (imho).