Last night I quickly drafted some ideas on IRC for you but you already left before I was done uploading. You might want to try a much more aggressive shading and use of tones. Also to shape things out: your trees have a kind of dark pillow shading around them but do note that when light is coming from the top you'll have to give the trees much more depth by making almost half of it, the underside, much darker than than highlights.
The same goes for the ground pretty much, you can easily use more of the darkest tile color to fill the undersides of the stones. The grass seems to have rather nice variety now, you could add more of the darkest color and make it feel like it's "flowing" on the rocks so it could look more separated from the rocky ground.
Anyway, it's getting there nicely!