Thanks to Scott for passing along a scholarly paper by Jefrey Frost at the U.S. Sports Academy titled "Characteristics Contributing to the Success of a Sports Coach."
In his research, Frost found that "in all kinds of sports, there are characteristics that successful coaches share," including "(1) the quality of practices, (2) communicating with athletes, (3) motivating athletes, (4) developing athletes’ sports skills, and (5) possessing strong knowledge of the sport."
Of those five characteristics, Frost's research determined that "the ability to ensure the high quality of practice... is a successful coach’s most important characteristic."