(I don't have any invites left btw)
Wave is kind of an interesting idea, once you get it explained to you. I think its biggest hurdle will be with people who don't know what it is. This isn't strictly limited to computer illiterates, of course. I've had some of the cleverest people I know (some are even Google engineers) sit down in front of it for a few minutes and go "wtf is this".
E-mail's been around a long time, and having to say "It's kind of like a re-thinking of email" doesn't really do is justice.
Think of it like this: What are the websites or programs you use every day?
Me, I use a webmail client, a calendar client, an RSS reader, an instant messenger, maybe a few ssh terminals (although I'm part of a dying breed on that one).
This is basically putting them inside a web browser all in one. It isn't right now, but it will be. It's probably the closest thing that's come to a Web-based operating system, completely abstracted from the hardware and with data in the cloud.
It won't get marketed like that, of course. It'll sneak up on you. You ain't seen me, roight?