I'm not sure why others aren't able to [take quizzes/etc].
Presumably it's up to the schools and professors actually giving the course. The UMich Python classes are clearly done by an Open Source/Creative Commons Evangelist (textbook, course materials, and videos are all explicitly labeled as OS/CC... Pretty cool, actually!)
(The change is that charging now seems to be the coursera-default policy.)
I expect some if not most of these [MOOC] courses to be freshman level courses at university
Most of the classes seem significantly shorter than an actual university class, perhaps with a full Coursera "specialization" being slightly more than a full semester's worth. I completed two of the UMich Python classes in about a month, though they were nominally 6 and 7 week classes. (A quarter-based university is about 10 weeks per class, and a semester-based university is longer. Usually about 3 hours/week of class.) The EdX/MIT Python class also nominally 7weeks, but was more rigorous (more lecture hours, more assignments, more "this will be something that you can't just race through.) (The UMich classes were more like those week-long training classes aimed at working professionals.)
I find the whole spectrum of MOOC styles and etc to be fascinating to watch, in general. I've taken (loosely speaking - ie "watched at least some of the videos of", although I've actually completed and passed several) more than a dozen online classes, and even the one that I'd rate as "poor" was useful. (and some were REALLY GOOD.)
BTW: I just paid for my daughter to take one online class during the "winter session" (about 1month) at the school she is attending (SUNYSB, out-of-state), and that was about $3k. So even at the ~$400 for a Coursera multi-class "specialization", it's a pretty good bargain! (OTOH, the daughter's class will earn credits toward her degree...)
online courses where a single set of video materials and articles can be reused for years to come
So far, classes are doing a pretty impressive job of revising themselves to incorporate new technology, redo questionable bits of lecture, and etc. You can see how much work goes intro producing some of the classes (especially compared to some of the early attempts where they'd just publish videos taken in a classroom.)
I supplemented a lot of my self study calculus and algebra with Khan Academy.
Indeed. I think it's potentially game-changing for students at a university to be able to go out and look at some other teachers' lectures and material to get clarification and supplemental knowledge. I've taken python classes from three different universities now, and it's an interesting mix (I'm sort-of looking for a programming class that i can recommend to my not-really-interested-in-programming children. Because everyone should know a little about how computers are programmed. But not all classes (and especially university "intro" classes) are aimed at anything other than being able to move on to the next class... (I'm still looking.))