I really like those, but you have a dark line on the far right of the tiles. Its more noticeable in the grass ones, where it is actually just a total absence of detail, but it comes back by the middle rock in each tile, you could probably fix that by (I normally really wouldn't recommend this,) selecting everything but the far right line of pixels, and stretching everything over 1 pixel, then fixing up any area that turned to crap as a result. The darkest tile set also looks off, if you are going for a gradient, you want to lower the contrast between it and the last shade, and if the idea is that that part is non-colliding and the stuff above it is colliding, you should darken it more to up the contrast, to make it more obvious, right now it doesn't really make sense, and If I was playing this, I wouldn't know if I could run passed it or not...
Totally aside, the grass itself looks awesome. Reads as grass, and isnt too obviously tiled.
