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.
Last update: Thursday, June 3, 2010; 4:01:08 PM
~About the Author~
Dave Winer, 55, is a visiting scholar at NYU's Arthur L. Carter Journalism Institute. He pioneered the development of weblogs, syndication (RSS), podcasting, outlining, and web content management software; former contributing editor at Wired Magazine, research fellow at Harvard Law School, entrepreneur, and investor in web media companies. A native New Yorker, he received a Master's in Computer Science from the University of Wisconsin, a Bachelor's in Mathematics from Tulane University and currently lives in New York City.
"RSS was born in 1997 out of the confluence of Dave Winer's 'Really Simple Syndication' technology, used to push out blog updates, and Netscape's 'Rich Site Summary', which allowed users to create custom Netscape home pages with regularly updated data flows." - Tim O'Reilly.
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). 
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