I think that TheFallibleFreind, Justine and Count Iblis II have more or less hit the nail on the head. I would like to add that Science is part of the human condition - a good part at that. To be a good scientist your mind must be "lit up" and very active with it. The same thing is true for teaching. I have noticed that students very quickly evaluate and rate professors, instructors, TAs, etc. If your a burn-out, stuck-up,ignorant or just a dummy they know it immediately. On the other hand, can spot genuine passion in a heart beat. If you know your subject well and can tell a good story you will captivate the student's minds. It is just like reading bed-time stories to children. Once you have lit the flame the student will home in on the subject like a missile.
If you are studying a subject and your teacher is killing it - you need to take a different tac. When I studied differential geometry, many years ago, the professor strangled the whole thing and then spent the remainder of the semester beating it to a pulp. I was forced back on my own resources. Essentially I "read around" the subject. I went to the library and looked up the history of the subject and all the punters that had added to it. It is quite a tale. I also looked in every textbook on the subject for "good explanations."
Looking back, I wonder if maybe that prof did me a favor?
