For 10u:
1) Coachability - kid listens, does what we teach, and parents don't try to interfere with what we are teaching!
2) Desire - kid puts in effort, shows how important it is to them to become a better player. At 10u, if they want it bad enough they can get there! This also affords that they will work outside of your practices, some 10u's dont understand the difference of how much work TB vs REC is... (give me a kid who dives for a ball, takes a bruise and shakes it off in tryout drying her eyes so you don't see her cry...)
3) Physical skills - do they have 'enough' physical skills to compete with the team / level your team wants to play? If they are coachable, and want to be coached, then the physical skills will take them to whatever level that you can teach!