When I did online schooling before my downfall was actually doing the work. It's all up to you, you dont have someone telling you what to do so you have to be very self motivated to keep up on work. It works extremely well for some and horribly for others.
One of the biggest obstacles might be a reliable internet source or maybe if the servor drops alot. It happened to me and I ended up failing the class because my work was not reaching my instructor.
Motivation. If there is no one there to see if I am paying attention, chances are I wont.