Firm handshake and eye contact. A smile (I'm not going to beat you, so don't look like I'm leading you to my torture chamber as we walk from the waiting area to the office.)
An ability to look comfortable while making small talk.
Genuine enthusiasm for whatever I ask about from the AMCAS application (athletics, hobby, job, research, volunteer work, elective courses).
A genuine, well thought out but not rehearsed answer to the question, "why medicine?"
A willingness to talk about how problems or weaknesses have been overcome. ("I see that you bounced back after a terrible spring semester in your freshman year; how did you do that?")
A good question or two about the school. Extra points if the applicant throws in a comment that shows some initiative in learning about the school before interview day. (please do not ask me what makes the school "unique". Unique means "one of a kind" and that is asking me to make a judgment about my school in comparision to every other school, something that I am unable to do)
Confidence but no arrogance or sense of entitlement.
No rehearsed or "canned" answers. A smooth conversational tone.
A handshake and a genuine "thank you" at the end of the interview.