Yes, it's quite sparse. In the entire world there's only one major river worth depicting? Are there no cities? Forests? You can probably lose a lot of the dullness by filling it with more detail.

I think you should reduce the opacity of your ocean clouds layer. It's overwhelming the map right now, but what you want is for it to just break up the monotony of the oceans rather than being a feature to itself. You might also see if you can reduce the size of the clouds by about 50%—increase the detail, then reduce how obvious it is. I think Gimp has controls for that (unlike Photoshop, which has a really crummy clouds renderer).

You might also reduce the spread of your coastline glow.