[aseembehl]

"is Udacity concentrating on short, relatively easy classes for mass appeal and Coursera concentrating on more difficult and rigorous classes?"

Absolutely, your inference hit the nail on the head. Coursera is trying to fill a gap. Very few people across the globe have access to high quality higher education.

Udacity on the other hand, though noble in in their cause, have considerably watered down lectures/assignments for greater accessibility. I can understand the watered down programming class for greater accessibility, since even limited knowledge of programming is something which can greatly empower people who had no knowledge of it before. But advanced classes like robotics are pointless if they don't go into depth of the topic.