You've set the density far too high for those igh clouds, they need to be much softer, it just doesn't look realistic as they are. i would much prefer a different colour to grey for the rock, because it's dull (especially when the only other colour is white) and it's been done a lot before. The displacement on the slopes is wacky, but in a good way. You need more colour in the shadows, I suggest using oshyan's or bigben's fill lights along with GI for this, oh and change the colour of the sunlight. Make it slightly blue (a cold snow picture) or slightly yellow (a warm snow picture) to give a bit more colour to the scene. Otherwise, looking great!