I recommend you to make a gaussian blur with an amount of 2-3 to your diffuse map. It helps smoothing the displacement very much. Otherwise you get too rough surfaces. Crysis has a clever displacement mapping on the terrain, all small rocks and stuff (the ones on the terrain only, not the big rocks which are modeled and placed separately) are displacement maps on the terrain but when you go away, they get thinner and at the end, they become ordinary textures when you are on enough distance. They can make samples of a small region with that technique, otherwise they had to fragment the whole terrain into small pieces, which is not possible in practical case.