I think Wave's lack of success is due to a lacking user experience.

Google integration hurt wave because it forced your friends also into that model. Etherpad had a much lower 'buy-in' that made it great for experimenting. The lack of folding messages hurt the returning-user experience - I've read the same msgs at the top of the wave since we started it. Outgoing email also seemed to really like to quote the first message on the wave, and never give anything more useful than 'the wave was updated', so I'd put the effort to login only to find the only activity was a non-committal 'uh huh'. Facebook at least tells me who messaged me.

I believe in Wave's innovation to realize I'll email out a rough draft of 'things to bring on the trip' and the other people on the thread can both respond as a new message (hey, whats the plan for food?), and also to add/remove/comment on my list of things to bring.