The Cavalier Daily's article Rate my MOOC discusses the differences between the standard bearer of student angst RateMyProfessors, and the creation of MOOC review sites, Top Free Classes, Grade My Course, and CourseTalk. The article discusses the inherent biases of the review sites, given the properties of the students. Traditional undergraduates usually use RateMyProfessors when they feel strongly about a course, usually negative feelings. I have found that it is unlikely that a student who passed a course, and enjoyed it will take the time to write a review. The drive to assign blame is unfortunately greater than the drive to offer positive feedback.
Students who use the MOOC review sites, the article claims, are those that strive through the course and complete it. For these students, the blame for not completing the course rests solely on them because there are no outside forces require them to take these courses. Undergraduates on the other hand have to take courses they may not want to take. This proportional relationship between being forced to take a course, and the resulting blame-passing (if you are forced, more blame, if you aren't, less blame) is one that needs to be taken into consideration for all educators. Teacher reform efforts inherently have this bias, as do for-profit colleges, and charter schools; students will rate a teacher/instructor/professor poorly if they are required to take a class and perform poorly. Just another reason why student surveys should always be paired with objective data.