I haven't been following the Wave hype, but by the end of the rather verbose first page I still had no real idea what Wave is supposed to be or why I'd want to use it. Best I can tell it's some mish-mash of email/IM/docs - which frankly sounds horrifying.
Unlike physical mail, most emails don't stand alone. They are part of a conversation. Conversations can get quite complicated. In mailing lists, they often get very long, effectively taking the shape of an HN thread.
Now, plain email is a mish-mash of message/IM/docs. We already use it these ways. Wave, by being very permissive, make these tasks easier. Single messages aren't harder, ML-like conversations have an enforced structure (easier to follow), and the collaborative editing thing just emerge from the possibility of modifying the entire conversation.
I see Wave like I see Lisp: a simple and consistent core, which allow many possibilities.