On thing that helps mitigate the problem quite a bit is that courses on Coursera are usually taught by professors of world famous schools such as Berkeley or Stanford, and sometimes the teachers themselves are famous. E.g. their functional programming in Scala course is taught by the creator of Scala.
I realized that the more famous the professor and the more famous the school, the less effective they were at actually teaching. Now there are definitely some gems, but in general, teaching ability is highly detached from research capability.
Now, all of these famous schools have dedicated lecturers that run the high-volume entry level courses; they are professional teachers and really focus their efforts on running classes. I can imagine future online courses being developed in a similar manner.