I generally just lurk but I got a strange urge to see what I could do with this. Mainly I just messed around with the palette.
First thing I did was change the sun to a yellow hue and make the AA colors lighter so it wasn't as rough looking. I also added a pixel to the end of the curves for the same reason. Then I changed the clouds to a more stereotypical white/blue. I decided to make the water a teal color because I felt it fit better with the rest of the colors. The rock I decided to make a saturated brown rather then a boring grey, I wanted to make the outline lighter too but I'd have to add colors. And finally I made the grass highlights brighter, though in retrospect I probably didn't go far enough.
I was going to try to fix the repetition too but I didn't feel confident enough to be able to copy the style down pat, plus I'm getting a little tired. I analyzed the tileset though and I think I see the problem. At first I thought it was 32x32 with isn't necessarily a good thing in this case. You have two different versions of some tiles to break it up but I think you just edited your original tile a bit and called it done. When I want to break up tiles I redraw the same kind of tile from scratch and see what I end up doing differently.
As for the emptiness I'm inclined to say just add more life to it. Plants, people, animals, a few more clouds, hills in the background maybe. Also adding more grass to the tiles instead of the 'moss' you have right now would help.
Sorry if this isn't as well written as it could be. I don't usually do this and it's getting a bit late.
EDIT: About making the rock's outline lighter it later occurred to me to just make it two colors instead of three.