I think the lack of a ridge line/peaks is the reason the mountain area(s) looks so "gloppy". I'm not sure about GIMP, but in PS I would create a new layer on top, then draw in peaks and ridges using a layer style that would emphasize the height. You may also want to choose a light direction and slightly shade the relief areas on the opposite side of the mountains.

Hyptosis mentioned farms. Are those farms on the coast? If so, they are HUGE fields! Is that intentional? Or perhaps I'm still having trouble figuring out the scale?

The forests look much, much better! Good job!