The cliffs are pretty decent, albeit organic looking (they invoke more of a wood texture than rock).  As for the ramps, they're suffering from a pudding-like look than generates high contrast as well as questionable color choice.  Perhaps start with a flat dirt tile and then think about the reshading pattern required to create depth.  Naturally, the reorientation for the various directions will affect the lighting/shading of each.

Note that "natural" ramps are kind of hard to draw.  Structures like that are somewhat rare in reality; most ramps are man-made and adhere to a rigorous and simplistic design without much of the natural variation that gives charm to certain art styles.  Perhaps for these reasons, a lot of games prefer to utilize only stairs instead of ramps.  If you continue to struggle with it, I'd say give stairs a shot.  Assuming you're dead-set on having Starcraft-like ramps in the tileset, maybe it would help to have a look at some photographs (or even better, 3D models you can load up and manipulate the camera and lighting on).