You have a decent start with the bushes--the texture isn't bad--but yeah, they're really flat. Picking a light source will help, and remember the shadows too. Also, instead of having a hard black outline, have the bush slowly darken towards the edges to convey depth, although the degree of the darkening will depend on where the light is. You need to understand the basic geometry of an object before you can portray it. You can abstract a bush as a sphere (or a bunch of spheres clumped together).
Okay, you see the bright highlight off to the side and how it gets darker around the edges, then has a tiny bit of bounce light on the back side. You want your bush to sort of vaguely approximate that. It might help to blob out the colors to indicate light source before you add the shape of the leaves.
As far as palette goes, highlights tend to be warmer than shadows, although this depends on material. You generally want your shadows to slope towards blue or purple or brown and your highlights towards yellow. I'd recommend purple-blue shadows and yellow highlights, and that applies to both the rock and the plants. You also want lower saturation than you have there. The green highlights you have now are sort of garish, and moreover, you usually want backgrounds to be less saturated or something than the sprites so the sprites stand out.
On a somewhat unimportant note, your background color hurts my eyes; I usually use a neutral color like gray (or white because GraphicsGale is kind of squirrelly about non-white backgrounds, but in an ideal world, I'd use gray). Yellow is also usually a good choice. It doesn't really matter what you pick as long as you like looking at it, but sometimes you can make better color selections with an appropriate backdrop.
This is a pretty good tutorial that covers a few of the basic tenets of pixel art and tiling in particular. Good luck!
