I dig it. Water and rocks look nice. The buildings look a little 'floaty'. Maybe if you ease up on the blur on the shadows and move them in closer to the buildings just a tad? Not sure what's with the grey you mentioned on the trees. They could benefit from a little tweaking to the drop shadow as well, I think. Maybe play with bevel on the tree layer to see if you can get them to pop up a little?

I don't know GIMP either so I can't be much help with specifics.

I think yours is looking pretty good! Glad you liked my maps. Have some rep for the flattery...erm, I mean, have some rep for the great map!
M