The biggest problem I see with MapBox right now is that their map labels don't seem to be dynamic. There are a lot of places around my house I can zoom in and I have to pan to see the street name because the label is out of view. There is one location nearby where I can scroll 3/4 the way across town and not see a label on one particularly major street. This could quickly make for a frustrating user experience.
Don't get me wrong, I applaud what MapBox is putting together and I'm sure it'll improve rapidly. At this point though it does seem like a significant drop in functionality for a major service to move onto it.