It seems like this is really "enterprise collaboration" done right. Think of how many places installed lotus notes or ms exchange to get some of these features.

Sprinkle in some calendaring and todo robots and you have an extremely powerful collaboration tool. Add in search and you have a new way to manage ad-hoc information.

It will be interesting to see how they come up with efficient representations when these document trees get to be large.

One downside of wave is that I haven't seen anything about how to stop spam. That was a major problem for email, and is becoming a problem on twitter too. Throw in differing social contexts like proper distribution of tweets for spymaster and you have some thorny social issues.