First off, what was Google Wave? Wave's goal was to improve message collaboration within a group. E-mail, as we currently know it has been a bedrock for network communication before I even existed, but like all things it has it's shortcomings. A big one is collaboration. Yes it brings people together but, at the same time it can cause confusion when you are dealing with multiple people sending out messages related to different points of an original message. Sure, it might start off just dandy with people throwing out simple replies, but it can quickly become an electronic confusion.
Google Wave had a better approach which allowed participants within a wave to respond to various points within a wave and even start a new wave based on an existing one. What I really enjoyed was the playback feature that allowed you to replay the wave from inception to the current point. That is something that e-mail desperately needs.