I'll be honest I had no idea who Sam Altman is so I read his wikipedia page and I still don't quite understand how he got to where he is today? Was his success started by the Loopt sale?
He's a very smart guy, according to a lot of people, and he's also nice. He made an app that was reasonably successful, sold it. When in YC, pg recognized his genius... and over time decided he's the guy who should run YC.