What are Google Wave's most powerful features?

The dragging and dropping between different applications

E.g. moving events between wave and calendar. I think I remember that. Maybe wishful-thinking induced hallucination.

Real-time updating of collaborative documents and discussions, to the extent of see what other people are typing, effectively merging email with instant messaging.
Emails are transformed into collaborative documents

Google Wave is open-source, so developers can make variants of it.
Waves can be embedded into a web page, potentially allowing blog articles, and the associated conversations, to exist simultaneously on the web and in your wave client.
Google facilitates Wave "robots" which are automated participants in waves, with the ability to automatically perform actions in response to certain events. For example, they might publish a wave to a blog, and then publish the blog version's comments to the wave.
"Gadgets" can be placed inside waves; for example games and mini-tools.