The simplest way I can put the Email/IM dichotomy is: it is file vs stream.
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.
XMPP might be the answer, or maybe dialects on top of XMPP. (haven't looked too close to see what's possible).
http://xmpp.org/extensions/xep-0201.html (last updated Feb. of last year)
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.