Playing time, timely promotions and coaching are key in development. If 10 people respond, you'll likely get 10 answers on how to develop players.
I want good coaches in the "main" category. Patience is a plus but it's all secondary to me. I plan for players to be in AAA when they're added to the 40. RL/LoA/HiA for the first two seasons, AA in the third and AAA in season 4. Sometimes, if a player is developing faster(or I need him), he may be in the bigs in S4. So he may skip a level. I pay no mind to coach recommendations on promotions.