My thought is that the color range (apart from the graphical things) is rather flat, not dynamic. The land has 2 colors, greenish/yellow and yellowish/green. The ocean has grayish/blue and greenish/blue for the depths (that seems kind of backward as shallows are usually greenish/blue). There are no real darker spots or lighter spots - it all seems the same tone. That's what I see, shrug, so take that as you will. I'd try to add some contrast with light spots and dark spots. When I've got a flat muddy map I just make some colorful clouds and mess with blend modes just to liven things up and give me new ideas.