The API, which "has 10 times as much traffic as the site itself," has been arguably the biggest reason for Twitter's success. I'd say its simplicity and the fact that it was more or less the first of its kind were just as important. Good article.
Open Social Web: A Bill of Rights for Users of the Social Web
Authored by Smarr (Plaxo), Canter, Scoble, and Arrington. Brief but a good summary of recent concerns. Comments interesting, but post needs a follow-up soon, and we can probably expect a lot of activity on this blog after DataSharingSummit.