"To live greatly we must develop the capacity to face trouble with courage, disappointment with cheerfulness, and triumph with humility."
I have had a lot first hand experience lately with leadership, and what it takes to be a leader. I have found the qualities that I feel very strongly fit that of a great leader, and admire those that possess those qualities.
First, you have to have the drive and motivation to be able to always set goals, whether big or small, so you can continue to make progress of your successes. If you do that then those small successes will eventually lead to big ones. Being able to continuously set goals for yourself personally, and professionally, is something I admire in others and try to always do for myself.
It's inevitable that you will make a mistake at some point in your career. How you handle and try and fix the situation, and learn from it, takes courage and humility. Nobody likes to be wrong, but being willing to own up to your mistake and learn from it, speaks so much of ones character.
And lastly, honesty and integrity. You have to be open and honest with yourself and want the best for not only you, but the people you work with. For people to follow you, they need to be able to trust you. That is one of the most important traits I think a leader needs to have and will always strive to have that in my leadership style.