Excellent break-down of the usage patterns of Email and IM. One thing that you hint at is persistence. While we all have the ability to log IMs, email exists as much for archival purposes as it does momentary communication.
The simplest way I can put the Email/IM dichotomy is: it is file vs stream.
Good call, that's yet another axis of communication. I actually had all that written down for a blog post I was going to get around to making (with diagrams & graphics and everything).
Something I'd really like to see if a protocol that is designed for notifications to humans. Anywhere from "Your car is ready to be picked up" to "We charged your credit card for this product, for this month of server", etc, etc.
I think if it is built from the ground up as a notification protocol, we can avoid the crap emails that we filter based on regular expressions, and bulk mail, and recorded phone calls.
But, it'd be really hard to get any sort of traction on of course.
Wave is XMPP, with additional Google extensions for a variety of collaboration features. That is possible because the first letter in XMPP doesn't stand for "XML", it's the (annoying but common) "eXtensible", suggesting that domain-specific additions are not only accepted, they are encouraged.
The simplest way I can put the Email/IM dichotomy is: it is file vs stream.