Dave Winer: Where is Twitter's WordPress?
I think azizhp on the comment area of that post answers it nicely:
actually, the Prologue Theme for WordPress pretty much fits the bill. Anyone who runs wordpress can use it. For a great example, see Talk Islam: http://talkislam.info. Note that if you run Prologue on WordPress MU, you can also use the Buddypress plugins and thereby get direct messaging and profile functionality too. its basically feature complete, aside from @replies (but that could be a simple plugin).
PS: I have no idea how to link directly to a single comment on Disqus-powered comments area. Anyone knows?