- Learning from professionals, industry/research leaders who can offer personal experiences rather than discussing other peoples' research/results
- University-level educational material for zero or no cost. A university education is a massive commitment of time and money, and in many cases the qualification itself, rather than the knowledge, is the end goal and thus the investment is largely lost if you stop partway through
- Convenience. Learn what you want, in your own time, at your own rate.
- Internet/forum culture. The communities tend to be cooperative and interested in teaching and explaining, and the atmosphere is positive. For people used to forums this can be easier to deal with than a classroom environment.
Overall, I find MOOCs tend to cover an excellent middle ground between the rigid style of teaching commonly found in established educational institutions and the individual learner mentality that educates itself by looking for tutorials, reading books, and learning by doing.
As to "what motivates people to take MOOCs?", they are convenient options for educating and furthering oneself. For some people, learning is a lifelong goal or a fundamental interest in acquiring new knowledge/skills. Learning can be useful for people looking to upskill in order to access new opportunities (self-employment, change of careers/employer). I recommend it to people who find themselves stuck in a rut (feel like life isn't going anywhere). Visible progression has positive feedback and can be stimulating.
Custom tag groups could be a useful extension.