Lately I have been trying to create a sierra style background. Heres one that I drew.
(http://www.2dadventure.com/ags/Owl.png)
The depth and composition look good. Sorta Sierra. Reminds me of the Wizard of Oz.
The image could use some contrast, some more darks. The roots of the tree are a weird shape and aren't really shaded/shadowed. The rock on the right is shaded alright, but it looks like the wrong light source. I imagine the light is coming from the center of the image, by the mountains.
The only advice I have is to use a smaller brush. Especially for the tree roots.
--Erwin
Don't rush it mate, take your time.
I don't see a sharp detail anywhere, I just see huge diffuse strokes. Like Erwin said, use smaller brushes and work out the details carefully. Use a reference photo to see what such a landscape can look like.