The best scene from anything star wars is when the Emperor hits the windshield during that attack run in the family guy version.^^
A ball makes a lot of sense for mobility, because you can immediately change course in any direction, rather than R2D2 who had to rely on his 3 fixed legs.Perhaps that ball isn't part of the droid, but the droid itself is only the "head" and he is balancing on a round object, moving it around.
Now this I liked even less, because this would be a hint that there would be slapstick comedy in this, like there was in the prequel trilogy.
I just thought that the design for it was stupid: an astromech head on that ball looks ridiculous.