Most of the best professors I have had were researchers teaching they favorite subject. Most of the worst were "CS education" types who should have been good teachers.
I had professors at my school that taught their pet research topics. They all sucked at teaching, partly because their research areas were utterly irrelevant for undergrads who were still struggling to learn programming. RDF, anyone?
The best profs were the ones who had been working developers in industry. They found ways to make the classes engaging and fun, vs just lifelessly reading their notes out loud (often in impenetrable accents).
1. Actually know something about, and be interested in, what they are teaching.
2. Being able to explain the subject without making a complete hash of it.
3. Being able to explain the subject without leaving me comotose in my desk-thing.
Extra credit for including tips and tricks that don't get covered in textbooks and are only known to people involved in the field. Plus, background and amusing stories about how the goofy nomenclature and such got the way it is.
On the other hand, when and where I was an undergraduate, if you were "still struggling to learn programming" after the first couple of semesters of programming and data structures (taught by aforementioned "CS education" people), then you were pretty thoroughly hosed. (There were, however, 1-course hour classes in individual programming languages, if you couldn't pick up those yourself.)
