I can tell you that the qualities I look for in a 2nd AD apply to the 1st AD as well.
– A calm unflappable personality under extreme pressure
– Real attention to detail
– The ability to plan ahead and think on your feet if the plan falls apart
– The grit to do a great job when they are sick / exhausted / just had their car stolen / got yelled at by someone above the line or any number of other things that would put a regular person off their game
– Sense of humor – essential!