Too much focus on Computer Science in my opinion. Udacity already covers CS in detail. The only interesting bit is the introductory chemics course, a focus on physics, chemics, biology and engineering could have set edx apart from udacity.
A lot of the computer science classes already have a lot of material in a ready to go digital format so it's easier to get them online. The others will follow, but it's harder to convert all the material from a "brick and mortar" class to a digital class of the same quality. There also might be a perceived need or demand for computer science classes.
B/c of Computer Science's current popularity, it would be a huge mistake to start an education initiative and not start with Computer Science.