Basic point: true real-time collaboration, the wiki way, plus coherent replies to an original message (subthreads, aka wavelets) and a timeline that maintains a complete PageHistory-like archive.
The official servers were shut down to further changes December 31, 2010. However, they are still accessible if you need to back something up from wave, at least for the present time.
They bought EtherPad? for its synchronous support, then cut the etherpad website loose. So right now the best bet for synchronous editing is the various sites set up and linked-from on [etherpad.org/]. Sadly, not quite as useful as Wave, but it'll have to do.
NatalieBrown: Wiki 2.0? Or is it too early to say that?
FridemarPache: Juanma, I am very interested to hear about your (ongoing) experiences with Google Wave and Google Search Wiki.
NathanielThurston: Also important: federated server model with an open-source code base -- anyone can set up a wave server, without permission from any corporate entity.
FridemarPache: Hi Natalie, this was a helpful page, you started. In accordance with this new philosophy of communication, I tentatively replaced "Discussion" by "Conversation". Motivated by your conversation input, I found and watched the following video: http://wave.google.com/. I was intrigued by the joint editing features, where several people could work on a document concurrently. I even noticed a concept-map, however it was not integrated into joint editing. But as the architecture is open to third parties, why not integrating it. As cmaps are collections of semantic triples that form a metric space, there are now even better ways than using forms for division of labor. That is: use cmaps. If realtime worked upon "propositions" overlap, don't touch them, work in a distance of some (linked) nodes away.
Pity it didn't work out, I guess. -anon