WhichMOOCsWorkBest
Surprisingly (or not), the quality of MOOCs shows the same trend seen in offline courses: the most significant factor is the quality of the instructor. Are they involved? Are they open to feedback? Can they change the focus or methods of the course to best suit the learners? Are they good communicators? Do they support community-building and connections between learners?
I've taken/observed mostly science-focused MOOCs, but I've heard/read a lot about other subjects and the trend I described above seems to hold. The down-side of this is that it's hard to quantify/predict quality, and information about the quality of the instructor is mostly passed on through word-of-mouth. Which also reflects a reality of offline courses :)