There seems to be a lot of excitement at the moment about the release of Google Wave. Unfortunately I did not get an invite so I didn't try it out myself. I read an excellent review on Lifehacker. So I have a good idea of how it works (although it is complex). I think that there are several great ideas implemented in the Google Wave product. However, the major issue that will stop its adoption is the fact that everyone involved in the wave must be using Wave.
I know you might think that this is an obvious restriction, but that need not be the case. Gmail was a radical overhaul of the mail interface. Although I am a big fan, I would never have switched over if I could only send and receive email to people also using Gmail.
I think Google needs to give some consideration to a mechanism to allow non-Wave users to participate in a Wave (e.g. by receiving email summaries and being able to contribute by sending email to a special address to update the wave.).