A system to push/pull selected messages to distributed servers (securely) would kill, and would be the basis of a revolution around the internet. I think Diaspora botched that chance, even though I sent them an email about it. Maybe they didn't really, but I hear their code is bad.

Besides, we need a platform, something like CouchDB, Diaspora is more of an app on top of the platform (but maybe I'm wrong on this one as well).

In a nutshell, easy/secure push would be a killer platform to build the new Twitter/Facebook/FourSquare on.

Of course, this couldn't be monetized easily, so maybe I'm wrong.