This will never replace forums, cuz the beauty of forums is you don't need to know everyone in a forum. Anonymity/random people are essential parts of forums.
Google wave is for your friends/acquaintances.
It does have some cool features, though. The group wave is really cool and could be useful especially if they bring this to smartphones. I'm not up on the latest instant messaging services, but do any of them currently offer group messaging????
With group messaging there also needs to be an option to remove yourself from a wave. He didn't cover it there, but that NEEDS to be an option.
The other great feature is the contextual spell check. That's a long time coming and should be available in word processors.
The bad: Well, you can copy a map directly in to a wave, but you can't copy a youtube video? That's weak, it should automatically be embedded.