Theoretical and practical skill - Knowing how the aircraft works instead of just knowing how to work the aircraft.
Reliability - turning up on time or early every time and getting all of the work done not just the bit where you take the pax from a to b. If not someone else on your team will have to pick up your slack.
Honesty - I have kept pilots on who have made mistakes but were honest with me (and themselves) about what went wrong while others who have made bulsh1t excuses or blamed others and they have not lasted long at all.
Attitude - It's been mentioned before and it's worth mentioning again Its partly the points I made above but it's a hard one to quantify maybe It's something like enthusiasm plus skill multiplied by the way you treat your work mates passengers and aircraft.