This goes the other way too. I signed up to teach a course and had close to 200 interested students. I was really excited, wrote up a syllabus and a couple small assignments (15 minutes tops) and created a wordpress blog for all my lessons. I put a fair amount of time in it and got a lot of great buzz, but that was all I got. 2 out of 200 people did the assignments and I only got a handful of hits on the wordpress blog. Noone commented on any of the discussions I put up. It was really disappointing and I ended up ending the course early.

That said, I love UReddit. Its a great idea. The reddit community is immense and there are a lot of opportunities for some really neat lessons that Coursera wouldn't necessarily offer.