Ok, here's a rapid fire list of constructive criticism. Just ignore anything that you don't care about / agree with.
1: It's weird that we can see individual grass blades half a mile away, but the closest grass is just a flat green surface with a few squiggly lines here and there.
2: The grass / plants near the river looks like thick aloe vera rather than thin blades of grass. Not sure if that was your intention.
3: I don't quite understand the way you've shaded these cavern holes, especially since the light is coming from above. The recesses wouldn't necessarily be dark, they'd just have stronger contours due to the curvature of the hill. Unless the light is coming from the side, there's nothing blocking the sunlight.
4: Your tree trunks look great and have three-dimensionality going on. They are bright at the center and dark on either side. I think you should use this principle for the tree foliage too. If you think of the trees as big green spheres, they would probably reflect more light at the center since it is closest, similar to the trunks (although the trunks are cylinders)
5: I assume the grass is WIP. If not, think about other ways to pixel chunks of grass. Look at other pixel art, for example.
6: The squirly lines you use to add texture on the grass don't actually tell my brain anything about shape or texture. They just seem random. They're not indicating light and shadows due to curves. If you're trying to think of ways to avoid a huge block of green colour, you could consider having a smooth transition between the nearest grass and the distant grass. The distant grass is pale and blue-ish due to atmospheric effect. Also, you could consider adjusting the texture so that grass chunks grow gradually smaller towards the horizon, indicating space and distance.
7: Same problem as above. The colors and shapes don't make sense to me.
I hope this helps. Hopefully others can step in if I've misspoke.
