I think Google Wave could become a big thing.
The idea of replacing emails with feeds of conversations is not a new one. I remember brainstorming about this with friends, and here is an old article around this idea from an ex coworker.
But the guys behind Wave not only turned this idea into reality, they are pushing it very far, with great ambition and with the power of Google. Wave might not only replace email and instant messaging: it may easily become the main user interface for many software systems.
One good thing is that the Wave protocol is open and designed to be federated, much like today's email (where you can send emails to users on different domains) and unlike popular instant messaging systems and social networks (which are centralized proprietary silos).
The key for Wave adoption seems to me a good Waves-email bridge. Imagine Google's GMail being replaced by Google Wave, but having good support for legacy emailing.
Update!: What a visionary! Google already killed Wave :-\