The MOOC fallacy is that "students" can grade other students; honestly, fairly, and competently. I bemoaned a course I took a year ago where some 10-20% of one's grade was via "peer review" grading.
Namely the concept is to take students, often not having English as a first language, or even second, and let them grade other students. Now besides the language barrier this means people who have no knowledge grade those with some knowledge. Furthermore what I also observed was gaming of the system by ganging up on those grades of what was clearly say Americans by those from other countries to suppress the American grades.
This year the course has raised the percent of the grade subject to this less than ethical practice to a level of some 36%. That means collections of students can target others and suppress grades for their benefit.
Anyone who has ever taught knows gaming. This system however verges in my opinion on the immoral. One subjects themselves to the rigor of the material only to have it grades by some one having no understanding of the material along with an interest in them succeeding and you failing. This is a wonderful example of a rigged game. It also in my opinion is an example why no one really trusts these systems.