I think we have to remember that wave, the UI experience, is simply 1 application built using the new platform/protocol. To me it looks like, in large part, a proof of concept that gets the ball rolling for devs, more than a surefire new product for end users. The google wave UI will evolve, and in theory other devs will offer entirely new visions built on top of the platform in more and more exciting ways. Wave's success will be more based on the platform success imho, than whether this early UI is a game changer.