You ask what you need to work on - well, it depends what you're trying to achieve. If you are going for "full-on" realism, then most things in your scene could be tweaked - I've been working on a file for over a month for the NWDA competition, and I'm still tweaking my grass shaders, for example; and because you have quite a close foreground, you could think about that too, and perhaps consider adding a grass population. But if you are trying to achieve a scene that you would like to look at, then you may not want to go to those lengths. As calico says, you have a scene that has both realistic elements (the rocks are good) and a definite CG feel too.
Incidentally, you wonder about confining fractal effects to specific regions - that can be done, usually by density or blending shaders - do a search on blending shaders.
