It's all a bit dark and murky, isn't it? For me there's no real sense of 3Dness about them. Good iso tiles have the quality of looking truly 3D as if they'd spilling out of their tiles. I'd suggest having the top-side of the grass tiles less regular so that there aren't so many straight lines.
And the trees, well there's too much darkness for me. It's like there's a little bit of detail sitting in the middle of a large lump of green. Sorry, but it doesn't really work for me.
The main things I'd do would be lighten things up and add some more detail to the grass tiles (and I mean detail rather than the slightly scribbly texture that the tree trunks have), give the water a more interesting colour with some depth to it and re-do those trees. Their shape seems slightly zelda-ish (ie, huge trunk) which seems add odds with the rest of the scene.
