Not like harassment complaints or anything (which I haven’t gotten but would personally take seriously and bump up an administrative level), just teaching kinds of complaints (which meansomething considers a litmus test).
Often my students complain about their other professors to me. These kind of complaints tend to come in two flavors: Ones where it’s obvious that the student doesn’t realize that the teacher is doing something for hir own good, and ones where I kind of agree with the student.
For the former, it’s easy, you just explain what the professor is getting at. R^2 is important when you’re trying to predict Y, but it isn’t important when you’re trying to figure out what the effect of X on Y is. Group work is unpleasant, but learning to deal with groups of people is important in many professions. Presentation skills are important and student presentations don’t mean the professor isn’t teaching the material. That sort of thing. Sometimes I’ll mention to my colleague of students aren’t getting something that they need to know and then the colleague gets bonus points from the students for going over it again in class.
The latter, when I kind of agree, is a little more difficult. I will sometimes sympathize and say something like, “I probably wouldn’t do well in that class either, but X is very good for other learning styles,” or “X does that so that you learn to learn on your own,” or even “Because X is an under-represented minority and a woman, she gets a lot more criticism for her teaching and has to keep tighter control of her class– Dr. Fullwhitemale can get away with things that she can’t, and he can get away with more than I can and I can get away with more than she can. People automatically give him respect, and I don’t have to work as hard for respect as she does.” Generally I try not to ever trash one of my colleagues even if I disagree with their styles.
Of course, my colleagues do take their jobs seriously. There are valid reasons for allowing or not allowing students to do homework in groups. There are valid reasons for different types of lecture/classwork modalities. I don’t hear about my colleagues failing to show up for class or never getting back homework (except in rare cases in which I can say that my colleague has been having a family emergency, which is totally understandable). I think in those cases I would probably just frown and not say anything. Because if one can’t say anything nice, one doesn’t say anything at all.