Out of curiosity, what class traditionally 'belonged' to that teacher?
Freshman classes for non-majors (think pre-algebra for art majors) taught in a harsh-but-fair manner is going to score differently than an upper-class or graduate level on manifolds.
One was an english professor who had control over the courses about science fiction and fantasy, and often taught a Tolkien course. No real mystery why he was so popular (although he was also a really great personable guy).
The other was a math professor that taught the standard assortment of undergrad math classes. I had him for Calc I, Calc III, and Discrete. Most of the math professors in my university that were teaching those classes (particularly the calculus classes) were either hated or unknown. However he had a reputation for being a truly gifted lecturer, and for having incredibly accommodating office hours. His reputation was well earned and deserved.
Michigan? Rabkin?
I never actually took any of those classes from him (very difficult classes to get into), but I did have a creative writing class with him freshmen year. He gave me a lot of really good feedback that made it clear that he had carefully read what I wrote. A very good professor in his own right, but teaching those very popular classes could only help.
His classes were notoriously hard, but he was so well-liked for four reasons:
(1) he was a gifted lecturer who put an immense amount of effort into finding clear explanations of every single concept -- nothing was spared from his relentless pursuit for clarity; he was also genuinely excited about the subject and was able to pass that excitement off to students.
I think students appreciated that, instead of putting his effort into creating some complicated class structure and a huge syllabus and fancy slides, he instead invested himself in finding the best possible explanations. There was definitely a sense that attending his lecture was a lot more valuable than reading the book, though that was always recommended too.
(2) he was very generous with his time and would always accommodate questions or requests for help.
(3) he took a keen interest in each and every student and kept up-to-date on everyone's individual progress; he never wanted anyone to accidentally get left behind. That doesn't mean that he'd give you a good grade out of sympathy, but it would be clear by the end of the class that you wouldn't have anyone to blame but yourself if you didn't do well. Even for the most jaded student, it's hard to be mad at the professor when it's made exceedingly clear that it's your own fault for not doing well.
(4) he was absolutely and utterly not pretentious in any way, and he was, in general, extremely kind; he had a way of making anyone feel welcome, even excited, to be in his classes, even if you weren't a math major or weren't very interested in math.
I guess we can rank teachers in three tiers:
(1) The lowest tier, consisting of teachers both loved and despised who either way fail to prepare students for future classes.
(2) Abhorred teachers who do succeed in preparing students for future coursework.
(3) The highest tier, consisting of beloved teachers who do succeed in preparing students for future coursework.
It's very hard to get into the third tier, because it requires remarkable finesse to make students feel good about having to work extremely hard and possibly still not getting a good grade.