I like the palette, but I think the brightest hue should be slightly more bright to be effective, as it is blending with the next closest hue and pretty much going to waste.
The tree is too triangular...I'd say try to make it more round. Also, if you can, try to vary your basis-spheres because at the moment, they all look pretty much the same--especially in size.
I like the rough/desert ground you've got going. I like the top-left square the best, because it is the least repetitive right now...but if you could get either of the bottom ones to look a bit less repeating, I think you would be in business
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As for the rocks: I think they look OK if they are the type you are going for, but the farthest back large one seems out of place, but I can't quite put my finger on it...it might be the triangle shape, the over abundance of the lightest shade, or the outline... but I'm just shooting in the dark right now (way to tired to investigate it further).
Good luck with your stuff, and keep pixelling.