He's extremely smart and extremely hard-working; he has personally, aggressively recruited others whom he's identified as the key people he wants working for him, and by all accounts he inspires them to work as hard as him; he's trained himself into deep domain expertise in both rockets and electric cars and serves as the chief designing engineer of his companies' products; but maybe most important, he has really extreme determination. Consider this exchange after SpaceX had lost its first three launches in a row:
Wired.com: At the end of the day you're still zero for three; you have so far failed to put a rocket into orbit.
Musk: We haven't gotten into orbit, true, but we've made considerable progress. If it's an all-or-nothing proposition then we've failed. But it's not all or nothing. We must get to orbit eventually, and we will. It might take us one, two or three more tries, but we will. We will make it work.
Wired.com: How do you maintain your optimism?
Musk: Do I sound optimistic?
Wired.com: Yeah, you always do.
Musk: Optimism, pessimism, fuck that; we're going to make it happen. As God is my bloody witness, I'm hell-bent on making it work.
http://www.wired.com/science/space/news/2008/08/musk_qa
Compare Paul Graham:
"I now have enough experience with startups to be able to say what the most important quality is in a startup founder, and it's not what you might think. The most important quality in a startup founder is determination. Not intelligence-- determination."
http://paulgraham.com/startuplessons.html