The main difference we find with the Sphero is the dynamic mast described by the patent. There is also a control system that keeps everything in balance, making sure the mast is always in vertical position. This, if you think about it, makes the whole mechanism behave like a miniature Segway. The arm can rotate and, according to the patent, magnetically interact with an external element. That means BB-8's head is most likely controlled via magnetic interaction. Magnets at the end of the mast and roller magnets at the base of the head.
Now let's use some 3D to show how the inner mechanism of the cutest astromech in the galaxy might look like.