Paul Graham on what he looks for in a YCombinator application:
We don’t look for any particular thing. We just look for people who have a habit of being great. Just so long as when they do things, they tend to do them well.
That and “be succinct; don’t just blather on ad infinitum,” he points out. Not unlike what Jim Coudal has emphasized about interviews with two candidates when both are equally talented, “I will always hire the one who can write.” So you know.