On paper, the interview went great and I had no problems with the whiteboard questions or any of the other questions. However, his body language was clearly not vibing with mine. He may have felt I came off too aggressive and overconfident. For the record, I did not think it was confrontational, in fact the conversation itself was pretty easy going and fun as far as technical interviews go.
I've been thinking about it and trying to come up with a way of calibrating my body language in the moment to make the other person more at ease. If anyone has any suggestions on reading, I'd love to hear them. I've read that FBI agent's book [1] since then and found it a little bit helpful, but nothing earthshattering. The author is more concerned with cracking/breaking people than making them comfortable.
[1] http://www.amazon.com/What-Every-BODY-Saying-Speed-Reading/d...