Starting out with tile graphics

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.net/page/resources/_/technical/game-programming/tilemap-based-game-techniques-handling-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.net/topic/606520-tile-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.files.wordpress.com/2012/03/grass.png which has very little detail, but has fairly high quality when tiled.

Kiwinuptuo has some of the best tutorials on low-resolution art, check out the one for rocks and tiles and you'll see what I mean. There's also the handy thread at the top of Visual Arts that has some good tutorials. If you want I can give you an in-depth explanation of tilesets with an explanation of the ones I've created for my current game, but I think these are enough to get you started. Good luck!

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.