The colors are very flat. Everything seems to be the same grey, brown, or green with different shades and it's all very hard to read. Try experimenting with different hues and saturation. Sunlight will actually cast a yellowish tint on some objects and leave blueish shadows. Even mundane things like vehicles and buildings have a lot of different colors you might not notice.
The sky becomes much lighter as it approaches the horizon.
A lot of the sprites seem to be have shading just because shading is some arbitrary requirement for objects to have, rather than actually adding depth to the object. Rather than just slapping a gradient on something, try to visualize it in 3d and imagine how light would bounce off of it. Shading is not easy, use references!
Objects that are part of the game should look bright (or dark, whatever makes it stand out more) and easily recognizable just from their basic shape and color. Background art should be non-intrusive and have less contrast and saturation.
You don't really seem to be using all of your pixel real estate to it's full potential on some of these sprites.
This guide will be more helpful that I'll ever be:
http://www.pixeljoint.com/forum/forum_posts.asp?TID=11299Btw, most of us start out exactly like you. You just need to practice a lot.
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