Aaron Swartz:

There are three questions you have when you’re hiring a programmer (or anyone, for that matter): Are they smart? Can they get stuff done? Can you work with them? Someone who’s smart but doesn’t get stuff done should be your friend, not your employee. You can talk your problems over with them while they procrastinate on their actual job. Someone who gets stuff done but isn’t smart is inefficient: non-smart people get stuff done by doing it the hard way and working with them is slow and frustrating. Someone you can’t work with, you can’t work with.

Good advice to start, and he’s got more where that came from. Some of it would be different for designers, but a lot of it holds for any position, at least in our industry.