I, for one, would not cherish having to draw John McCain for 4 (or, gasp) 8 years. Obama is a MUCH easier draw. For instance, Here's a quick sketch I did to illustrate for someone just how easy Obama is. Physically, the geometry of his face is just so much more exaggerated that it makes it very identifiable with just a few strokes.
McCain, on the other hand, has plain, more subtle, Caucasian features that make him look like just about every one's uncle.