A lot of the material on coursera isn't that easily available for free. The courses usually take a new and integrated aproach that you would see in a university course, created and presented by an expert instructor. I don't care for their prestige, but probably coursera is using the instutional prestige as a selection criterion, and it works, mostly.

I also think Udacity is more in between Khan Academy and Coursera, in that Udacity tries to find new ways of online instruction.