Agree about the displacements - are you working with a DEM here, or is this a fractal terrain?
The ground cover is nicely drab and well constrained.
The fake stones are very nice. The darkness of the lower left part of the picture could do with brightening - either through altering the sun angle/heading you have, or adding a second sun (a typical trick) with a much reduced brightness and no visible disk or shadows. Another alternative is to add another Environmental Light, with Ambient Occlusion - it's not a technique I've really worked with myself, but others could advise you.
Keep at the testing - it's how we all learn!
