It's the API
Wednesday, July 04, 2007 by Dave Winer.
Nik Cubrilovic: Twitter v Pownce: It's The API, Stupid.
The API is why Twitter is a coral reef, and Pownce is just a shipwreck.
Twitter, to me, is a very thin app, on top of a bare-bones identity system with a clean and simple API, very lightweight (and therefore not very secure, not suitable for banking applications, for example). And for not very much more complexity it's also a pub-sub system (not IM, though many people confuse it with IM).
It's one of the 25 most interesting pieces of software I've ever seen, not because of its technical prowess, rather because it is not technically impressive. You can master it as a platform in a matter of days. With good sample code (which I didn't have) probably in hours.