Wave is not really a replacement for email. It does somethings very well but clearly the focus is on collaboration.
If email were invented today, it would look more like TrulyMail. Here is a company which re-invented email. Sure they have few users but that will change with time.
We all know the problems with email: You don't know if/when your messages are delivered, too much spam, your messages are not really private, etc. TrulyMail solves these problems.
The only problem is that it is a complete replacement for email (it actually has nothing to do with email and uses its own servers). Their latest release includes integration with email to make the transition a bit easier for those of us who live in the real world.
Still, I'm looking forward to Wave going public. I hope they don't keep it in beta for seven years like gmail was.