I hate to admit it, but I signed up for a few courses at some MOOCs and never did even one lesson of some of those courses. I signed up thinking it would be awesome to learn the subject I signed up for, but then when it came to the time to do it, I didn't have the time to do it. That being said, I know that if I'd paid for the course, then I definitely would've done the courses or given more thought before deciding on whether I could do the course. The way it's all currently set up, you don't have to give any thought to it before signing up besides being excited, because if you change your mind later or if you can't do it later, then you don't lose anything for it.
That being the case, I think MOOC's should charge a small fee from everyone who signs up. Maybe $10-$100, depending on the course. When the student finishes, they either get all the money back or they get a percentage back. Then the student has something to lose, and will be sure to put more effort into the course and make people really think before signing up.