I like the mountains on this one. I really like the passes through the mountains, except... Well, the passes are kind of flat. The quality of this map is so great that it has inspired me to look into how to improve those passes.

As you noted multiple passes with the Difference Clouds filter over a Clouds filter makes excellent looking mountains. Kind of like a cross between billowy and ridged perlin. Nice. Unfortunately, they aren't very controllable. The photoshop Clouds filter has a particular frequency and roughness(not much). Which is bad, because what I want is another layer of relief below the mountains, basically a longitudinal hump with a bit of, still fairly low frequency, variation. I think I might want a gently rolling structure to that variation giving rise to foothills below the mountains. Once I have that I'll add it together with the mountain heightmap(using Linear Light mode in PS, I think Gimp just says, "Add"). Before adding them together, the two layers should be scaled using levels so that the sum doesn't exceed 255(at least not by much).

I'm stoked.