Perception, experience, tenacity. Know when to lead and when to follow. That's the most important quality in a good leader. Effective team leaders realize they neither know all the answers, nor can they succeed without the other members of the team. Leadership is the wise use of power.
Power is the capacity to translate intention into reality and sustain it.
Then follow through. Ultimately, leadership development is a process of self development. A successful team with 100 members has 100 leaders. Truly great leaders spend as much time collecting and acting upon feedback as they do providing it.