
Now, notice how steep your cliff(?) is here. It should be more gradual. Also, notice you have some grass tiles underwater, grass can't grow underwater.

You use these leaves really bad here, I'm really not sure why you did this.

Notice the jagged looking areas in what is supposed to be a nice little river, smooth them out with brush size 5 and smooth tool.

The poison tiles here not only look really ugly the way you used them(being so many), but really don't match any of the tiles nearby, and the bridge also looks too green and out of place next to the grass on one side, but looks just out of place next to the poison.

A tree really wouldn't grow right there like that, and if it did the water would kind of diverge around it.Also, the water goes directly to grass, and then a cm or two away is the actual water, fix that. And the waterfall seems to come from nowhere....there's no river to create it, just falls...
Additionally, the game actually lags a tiny bit when I scroll over some of those masses of weeds. Scale them up and delete some, or just delete some.