The inverse square force is extremely common. Plus, I doubt you use the permittivity of free space constant and so on, rather just the inverse square property.

You're absolutely right. Gravity is a force similar to the Coulomb force in this regard. If I were to swap the electric potential for, say, a hilly landscape where the ball is free to roll, then the ball would behave in an identical manner where its acceleration is proportional to the gradient of the height (i.e. it tends to roll downhill); just as how in the electric potential it's proportional to the gradient of potential.

Nonetheless, I like electric potentials. The idea for this game came from my earlier attempt Electric Potential Golf (www.dllu.net/em), which was in turn inspired by Electric Field Hockey (http://phet.colorado.edu/en/simulation/electric-hockey).