Passion

I remember when I was in Physics one time and we got our teacher sidetracked (very easily done by the way) but anyways he began teaching us college and research level physics, but in a way that made it easy to understand. His passion for this subject was so much clearer than that of what we simply were learning from the textbook. His passion for teaching made all the students in the class more excited about learning and made the classroom a much more productive place.