Udacity's model is to provide self-paced classes with absolutely no deadlines or time windows, as opposed to Coursera and edX's more traditional approach. I can see the argument that some people would prefer having more structure; that might help with motivation. I think the ideal platform would allow, like Udacity, the ability to start a class at any time, but provide options to tweak time constraints and deadlines for people who would like to include them. If sufficiently powerful and flexible, that might provide the best of both worlds.