+1 to what gjello said, though I would nuance #4 a bit. Patience seems to be a very important attribute for coaches who work with young guys (above all in the minors). I believe "Discipline" is equally important, for all levels, and above that it is what helps seasoned big-leaguers get better in their mature years.
In my world, a good fielding instructor is the hardest thing to find. They typically cost $4-5mil to sign, but only about $750k in subsequent seasons. So the loyalty rating to which gjello alludes is important in keeping your budget down. I believe WIS has said, at some point, that a coach with a 50 loyalty rating will re-sign 75% of the time.