EDIT: note that I wrote this message before you posted immediately above it. The grass shading appears improved since I wrote this.
First: your colors definitely need work. I feel r1k overdid it and changed the vibe entirely, but the basic point, that your colors are oversaturated and this makes the tiles hard to look at, is definitely something for you to take on.
Some other points:
* Your tiles are very similar: you are copying shapes with very little modification. This makes the fact that they are tiles more obvious (generally a bad thing as it leads to a feeling of fakeness)
* shadows on the rocks are inconsistent. these large lumps of rock cast shadow not only on their other sides, but nearby rocks as well. Some self-shadows (shadows that one surface of an object casts on another surface of the same object) are also missing.
* the dimensions of your tiles appear to be 18x18. This is an odd size and may not lend itself well to actual use in a game. Ideally tile dimensions should be a power of two, eg. 16x16, 32x32. If that's not possible, second-best is power of two plus a half (for example 2**4 == 16 + (16/2) == 24 -> 24x24, or 2**5 == 32 + (32/2) -> 48x48). These choices are easier and faster to program for.
* I'll back up what cheetah is saying: the grass is essentially pillow shaded. However there are more options than what he is mentioning IMO.
Here's a quick edit changing some of those things:
