WARNING: Just thinking out aloud here, nothing groundbreaking-

Twitter really leveraged the concept of an Asymmetric follow . i.e. I can find people I’m interested in, irrespective of our relationship, and learn from what they have to share. Similarly, Others can include me in their lists. It mimics real life, i.e. there are asymmetric levels of interest in all our relationships.
It also mimics real life since friendship grows with complete strangers over time only by communicating with them. In fact, through Twitter’s wonderful system of @ messages I find myself having conversations with complete strangers so many times. Most humans react positivley to attention, so we follow our @ messages more faithfully than our actual twitter streams.

Now comes the thinking aloud part:
Since the users have been warming up to the concept of lists, could twitter (or any network app) as a next step allow users to send directed messages ? For example: I have two very distinct set of followers on twitter 1) People in Bloomington 2) Techy/geeky friends from all over. The B-ton folks might like what I have to say about B-ton and everything else, but my techy/geeky friends from everywhere else might not be interested in what is happening at b-tons @bluebird bar tonight. Since there are specific lists that I follow, for example the really well curated list of Bloomington tweeple , I could plausibly @message the list & it would only show up in the twitter streams of my followers from that list . Or, since twitter already allows users to mention their location, I could @~ or @# a location ( e.g. @~bloomington or @#bloomington,IN ) when I tweet.

There could be many more interesting ways of using this, without making it into a twitter-HTML . Facebook already tries to do the same in very confusing and inefficient ways (my opinion). I hope Google buzz would have something on these lines. (This chain of thought started when I heard that google buzz, which mysteriously has not yet (1am,feb 10) shown up in my account, had mimicked twitter’s asymmetric follow.)