Forums, Google groups, etc, don't have the "edit other people's posts" Etherpad-like collaborative editing. Most of them aren't branchable in the same way Wave is. They generally don't have the privacy controls of a Wave. They don't weave from email-like to IM-like like Wave can. They're not built to allow native clients to interact. And they don't have any federation ability.

You could say that Wave is a bit like Email, Basecamp, Campfire, Etherpad, and a bunch of other collaboration tools, all merged into one unified interface. I'd be scared if I was 37-Signals. From everything I've heard, the feature Basecamp users most like is the discussion system... with Wave, that becomes a lot less worth paying for.