I'm currently taking a physics class, and my teacher explained that friction is the result of intermolecular interactions, and that energy must be applied in order to break these bonds.
This got be thinking about what kinds of things have a high coefficient of friction, and immediately my mind went to sandpaper. The atomic surface of these mineral crystals would be really smooth and not easily polarizable, as they are held tightly in place by the other ions. So does surface area have more to do with friction than actual Van der Waals forces? This would also seem to explain how geckos are so adept and clinging to things like glass, as the tips of their fingers are so highly ridged.