Not sure what to make of this yet. My initial reaction was....didn't this used to be called AOL? The more I read, I can see the differences but I guess I'm not sold that this will be what they hope it will.
It's supposed to combine email, im/chat, blogging, and probably some other stuff I've forgotten from the demo I watched. It remains to be seen whether it will catch on outside of a hardcore fanbase, but when they are finished, Google has promised to make the source available to people that want to setup their own Wave. That means it should live on in some format even if it bombs and Google drops it like a hot potato, which seems unlikely.
Collaborative editing seemed to be the biggest difference I saw between Wave and anything else. You start working on a document and other people that you have allowed to participate in that document can *simultaneously* see your edits and make edits of their own. In the demo, they had three or four people in different locations creating a single document and they could see, keystroke by keystroke, everybody's input immediately. It was kinda neat, but I'm not sure that a large number of people work that way.
Other than that it seems to be an integration and re-imagination of the interfaces for the stuff mentioned before. I didn't sign up for an invite, but I'm watching with an interested eye.
I don't know if this plug in will work on the Windows version of Safari, but Inquisitor (inquisitorx.com) is one of the best plug-ins ever both in terms of look and functionality, and is the only thing that makes me ever use Safari over FF on my G4.