I LOVE the shading on the trees/leaves. It reminds me of the folder icons from Windows 98, somehow. And the background rocks look great, too. They have a really simplistic style to them.
And yes, I would agree with Talps in saying that the foreground rocks do look a bit flat. Maybe use the same shading method used on the leaves/trees, but with a bit more depth?
(I also feel that the grass on the foreground rocks maybe needs some work as well.)