Well, there are a lot of reasons: the simple ones being he understands the importance of technical skills in startups, he's mature beyond his age, and he is smart and articulate in a focused way.

I think these resources help you understand why he's so cool:

http://tusb.stanford.edu/2007/05/sam_altman_founder_of_loopt...

http://www.omnisio.com/startupschool08/sam-altman-at-startup...

http://news.ycombinator.com/item?id=95857