When I was in university, I had a professor for a geometry course that all math teachers were required to take. I found it interesting at the time that he was a professor that students seems to really like or really dislike, in terms of teaching style. At the time, I chalked it up to his habit of bringing in ideas and skills from related courses, and the lack of good mathematical understanding by some students, which, I thought, should allow them to integrate the different areas of mathematics.