Since I play around with GIMP only for a week or so I'm a bit insecure about advice but:
I think you should blur the edges of your mountains a bit. Just like you blurred the edges of the woods.
Use an other Bump map on the woods than you used on the plain. It looks as if they are bumped in the same way (or as if the woods aren't bumped at all and the plains are looking through)
The Bump Map of the Plain also is seen on the lake.
The rivers are looking good, the only thing I think is odd that the river in the north seems to flow from the sea to a mountain lake.
Oh, and the little forest in the middle of the map seems to grow in the sea and the forest in the north hides the little lake.
All in all: Play around with GIMP, move the layers a bit and blur the edges. Try different types of Bump Maps (it's really fun to play with the filters and try to bump your layers with the results)
Oh, and did I say already that I like your rivers?