You need some variety on the fake stones do them decreasing size first.
i think you should work more on the background too, the mountains need more powerfractals of several different flavour (that is a ggod exercise to understand how they do work too)
then for the clouds, you should read the tutorial by Frank on them, it is a very good start and i couldn't explain better than he already did
here.
I think maybe you should work more on the basics of the scene (terrain and surface) before getting too deep in objects tweaking.
it is , IMHO, harder to do a good scene with good objects but bad "landscape" than to do a good render with good "landscape" and medium quality objetcs.
anyway, that's my advices and maybe others won't agree with them, but that's what i'll do if i were you
btw, i am french as you are (aren't you ?) so, if you want some more explanations in good old french, send PM or mail.
