Really, though, I need to do some experimenting here, to get a better idea of how it works, and Wave is fundamentally about multi-user, particularly real-time. The high concept (which is much clearer now that I can get my hands on it) is that they've re-envisioned email as a real-time, interactive process. The UI is reminiscent of Gmail, but the "messages" are live, interactive objects that everybody involved can update and modify in real-time. It doesn't appear to be principally a conversation system, or a community system -- rather, it's a co-editing system that happens to have realtime chat embedded in it. (Seriously realtime: the chat is part of the realtime Wave, so you see everyone editing their chat messages as they do so.)
So: who wants to play? If you have Wave, and are interested in playing with it (especially tomorrow, when I'll have more time), please reply or drop me an email. If you are really interested, and want an invite, tell me, but please don't do so *too* casually: I only have a modest number of invite codes, so I want them to go to folks who are going to really kick the tires hard. It appears to be particularly relevant and useful if you have stuff you want to do collaboratively in real-time online -- probably doubly so if you are already used to GMail and Google Docs...