Your approach to user signup is superb.
There should be an explicit "tag this conversation" link. Otherwise nobody will even know that they can do that.
Can I post a link to an ongoing conversation in some other place, like Twitter or, say, here?
The "Theory" page is... a bit too theoretical. I mean, everyone here will get it, but not the mass market. I doubt that many AOL Chat users understand what "asynchronous" means. Don't delete the existing page... just make a slightly simpler, shorter Theory page and relegate the existing text to a second-level "Theory of the Theory" page.
Other than that... it's hard to know what to critique. Many of the essential design features of such a site will only become obvious when it's flooded with traffic or overwhelmed by griefers. That's hard to test. Maybe you should start a really provocative conversation and then post a Digg link to it. ;)