Your terrain could perhaps use some more contrast; brighter highlights and deeper shadows to make the mountains and cliffs and hills pop up. Also, the mountains themselves appear to differ in style from the cliffs... which makes the map harder to read; I can't tell if the mountains are supposed to be read top down or on an angle, if the cliffs are supposed to have vertical faces or slanted. If the mountains are meant to be read on an angle, they might benefit from pulling some of the higher peaks above the range behind them so that you can't see the backsides of the mountains.

For the marshy valley (dark area) wide open flat spaces would be more likely to see a ton of light from the sun because there's nothing to block it, so definitely lighten it up. If you want to make it appear as a depression then pull the terrain up all around it (shadows touching the western side of the depression and highlights in the east and north, according to your sun position). The mountains on either side of the strait in the upper left corner is a pretty good example of that.