If you want a conversation between more than two people, Wave kicks everything's ass. And unless you're the kind of group that's comfortable with setting yourself up a phpBB, I can't even think of anything that comes close.

Wave's problem wasn't that it's not useful, it's that nobody knew about it (of people I know, it had 0 adoption outside of computer geeks), and it couldn't integrate with desktop applications. A lot of us are stubbornly hanging on to Mail.app and Entourage because they integrate with our workflow better than web based solutions.

If google had come out with a a desktop Wave client that worked with OSX's address book, photo libraries, etc, it would have had a lot more draw. That sort of integration is less expected on Windows, but I'm sure they could have come up with some compelling features that the website didn't offer.