All of my terrain layers are always shiny and smooth, I cannot change this with the smoothness slider (although I can alter the value of the smoothness slider, the smoothness does not appear to change in editor or game view). Additionally the Unity devs included this awesome feature where some terrain layers have a smoothness slider, and others have the smoothness controlled by the diffuse alpha channel. Which of these methods (neither of which seem to do anything as there's always a full specular, so cool!) applies to which terrain layer seems to be completely random. Awesome!