I think you'll find that with a technology like Wave it needs to do everything at once, or it won't do anything at all.
It will replace many things [blogs, forums, some IM and even Office Suites] and will be the paradigm in computing but only because it can replace everything. Once a user gets used to using Wave they will want to use it for everything that they do.
This seems obvious to me, as soon as I saw the video. I don't think I'm blowing smoke up Google's arse here when I say they've nailed the next big thing right on the head and everyone will be using it. Microsoft is dead, long live Google!