I liked what JavaFan said. Something I like to hear is the care and concern a hacker is excercising while doing a problem on the whiteboard. You misused a built-in's syntax a little? So? That's 90 seconds with perldoc to solve. You wrote something without discussing why it might be better another way or needs to be written as XYZ to be unit-testable? Well, that can't be fixed without a LOT of coaching, wasted cycles, and bugs that could have been avoided/caught by a more conscientious but less technically skilled hacker.

You're not just getting grilled for problem skills. You're introducing yourself and how you would fit in. Someone who is aware of and concerned for best practices, right tools, and even general community etiquette/idioms is, to me, a better catch than someone who has all the sort algorithms memorized.

Not to say that is what your prospective employer is looking for. :)