If I try to deconstruct your question, you are asking for methods / frameworks to evaluate effectiveness of learning tools (except in this case the mode of delivery is precised). So, in my impression you might want to look into ways of assessing effective educational practices. There are different models (for example instructional design theories (David Merril, Jeroen Van Merriënboer, etc.)) that might give you a framework to do that. I can imagine (based on your background, that you want to do something with the technological or media component of the environment. There are different multimedia theories for learning (Mayer) that might help you with that.
My advice would be not to look for specific theories to tackle the description of MOOCs, but to look for solid educational frameworks to analyse the effectiveness of instruction.