First off, let me say that your trees look absolutely perfect to me.
For the ground layer, have you considered doing actual ground textures? There are some very good ones out there that would help break up the smoothness. The colour is pretty uniform, I think that's what's standing out to me - right now everything looks like a well manicured lawn (who do you have maintainign that? do you have a card? the lawn outside my house could definitely use this treatment! lol). you could create another layer overtop, render clouds on that with 1 shade green, 1 shade brown then set that to overlay or something similar, that would add some randomness for you.
For the sawmp, you could create another layer, do the same thing only use a few blue tinted areas, and just paint over the areas you want to look swampy - and maybe adjust the trees a bit.
For fields and roofs and such, you might actually want to check out Ascension's city building tutorial, it has a few good pointers for that. In particular, for the fields..
It's a bit long for pasting, but you want steps 131/132. There's some good pointers in there overall for a map like this that you can warp to your own ends.
I'll look for the link. This might take awhile
[edit - or not!]
[Award Winner] Photoshop Town Tutorial