A coach must possess broad experience. She must have proven herself over time, work successfully with a variety of clients, each with their own special challenges, in diverse situations.
A coach much provide encouragement and support. She may assertively challenge clients to stretch themselves, to push beyond self-perceived limitations. But the goal is to strengthen rather than tear down. The personal styles of effective coaches may vary tremendously, but their intent is always the same: to empower individuals to realize their goals.