I'm not sure of the mood you're going for in your game (both of your images use fairly bright colours which give me a sense it's a rather "happy", lighthearted game, but that's neither here nor there), but the rocks look great asides from the grid (which you've already noted).
I found this http://www.gamedev.n...ing-terrai-r934 little article to be a really good resource when first starting out with tile design. You may have seen it already, but i think it's pretty good quality and needs to be mentioned regardless.
As for grass, try to use less detail, unless your game is on a very small scale.
Also, what's the angle of your camera like? The more top-down it is, the less definition you should see for each blade of grass, while a side-scroller will have more definition and something in between will have something... in between. A very quick search on GD brought me to http://www.gamedev.n...le-transitions/ which has a couple good examples of grass and the change in detail you see based on scale (though not so much camera angle). Try something to the tune of http://alastaira.fil...12/03/grass.png which has very little detail, but has fairly high quality when tiled.
The plan is for the overworld to be fairly bright; the GBA screen seems to dim the colors a bit, too, so I need to compensate for that. I think that I'll leave the grid in the rocks, but I'll move them to a dungeon level and use a dirt texture for the cliffs to create a warmer, richer atmosphere above ground. I should experiment with some less detailed grass; my plan is to have a fairly typical top-down perspective. I'll probably also make some grass with longer, curved blades to provide some variety.
I'm considering whether to switch to a 3D platform (while keeping the top-down RPG look) or to stick with the GBA. I don't really want to waste good hardware, but wasting time on something few would play is probably worse. Anyway, that's kind of off-topic, but it does influence the direction of the art. I'd probably keep the rock texture, though; I like it too much to scrap it yet.
...On further thought, the added ease of development has almost completely convinced me to do it on a machine that doesn't require me to manually push bits to the graphics hardware. I need to think of ways to take advantage of 3D acceleration while still making it look like a top-down tiled game. Some mirror and shader effects could look very nice...
Anyway, thanks a ton for the links! They definitely look like they could help me get better with pixel art. I'm not much of an artist (partly because of lack of experience), so I can definitely use help when I can get it.