Wave melds e-mail, instant messaging, document creation, and more into one coherent environment. There's plenty of feature-candy too, like:
- document creation functionality has notion of branches and merging as well as inline comments (that can be stripped on 'publish')
- context-aware spell-checker that can determine if you used the wrong correctly-spelled word
- automatic translation of text from one language to another
- the ability to split a portion of one message into its own thread within a wave; "inline replies" get their own thread
The product team did a great demo launch at Google I/O, you can watch it on YouTube: