He primarily taught the Honors Calculus sequence, but would occasionally teach other courses as well. Many math majors went straight into analysis, so a good portion of his students were engineering and pre-med students rather than pure math majors.
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.