First off, a good coach (in most cases) should have played at least at the high school or college level in order to have the proper feel for the game. He doesn’t necessarily have to have a reputation as a great hitter however.
A good baseball hitting coach must have outstanding teaching and communication skills. A background in teaching/coaching is probably preferred. Keeping things simple while displaying patience is essential. Knowing how to take step by step approaches to help build a hitter’s confidence and skilsl level is essential.
A baseball coach must have training skills and understand how the body works efficiently in the swing. Knowledge of how to trigger certain hitting muscles and an ability to fix bad habits while teaching effective mechanics are key skills in baseball.
A good coach must stay open to learning more about hitting and carefully listening to feedback from baseball players to make needed adjustments in the learning curve. A big ego as a hitting coach will ruin relationships with players and inhibit further professional growth.
Most importantly, the coach should have a track record of success as a hitting coach (not as a player); with lots of great references from coaches and players about how much the coach helped get hitters to hit better. Too many coaches are hired because of what they did as a player or who they know. This is called the “good ole boy” networking system and isn’t a great way to hire a hitting coach for serious ball clubs.