It's funny. Lots of companies are going to increasingly great lengths to filter candidates based on inane tests that supposedly measure technical competence, but many if not most of them do little to filter candidates based on their apparent interest in the job opportunity as a whole.
Obviously, there are reasons candidates shy away from asking "tough" questions other than "I don't really care", but startups in particular should be wary of candidates who don't demonstrate a certain level of curiosity beyond the day-to-day technical rigmarole.
Put simply, look to hire people who understand that interviews go both ways.