While "average person" is not the term I'd use, the question you're asking is a bit loaded. What measure is used to say a MOOC "works"? If we are saying completion, then high drop out rates are an issue. If we say high interactivity among those that don't drop out, this is where the current field of research in MOOCs is at. While I do not study it, colleagues in my lab are.
My point is that quantifying the effectiveness of a MOOC is an ill-defined domain. Instead, we acknowledge the attrition rate and do analysis on those that pursue the course. Current research looks at system interactions, the social networks of MOOC forums, etc. to identify student behaviors that show higher "gains", be it course completion or grade.
If a student leaves a MOOC, there is no way to identify why they leave. Likewise, unless asked, it is hard to identify why a student joins a MOOC. Students enrolled in a MOOC can come from a variety of culture, geographical, sociological, motivation, etc. backgrounds. As such, assuming traditional student behaviors is ill advised. However, in the process of learning, access to material is not enough to learning the material and additional effort is needed by the student to build the necessary mental models in their head. If a student drops out of a MOOC, it is hard for the runners/analysts of the MOOC to appropriately say the student learned the material.
As austenallred mentions, current education research is looking at the motivation of learning, as well as what constitutes motivation (and learning for that matter).