In a reply to my earlier post about Wave, Paul of the PC Thoughts blog posted a comment with a few questions about Wave. I decided to reiterate and elaborate on my response here:
I think Wave is pretty fun. Sometimes it's buggy, and doesn't do what you expect, but overall, it works as advertised. I've found myself using the "with:public tag:[whatever]" search somewhat frequently. Basically, what this does is finds Waves which have been made public by inviting the "public@a.gwave.com" user to the wave.
This makes Wave feel a lot like a forum, or IRC depending how active a wave currently is.