What Nathaniew said +
-the grass texture is way too bright in compare to the ground(terrain )texture beneath it and it makes it look strange.Maybe darken the grass texture more to darker/brownish green?
-play a litle more with the sculping of the terrain.Make more natural deformations and even small ones
-scatter small rocks on the ground.
-more trees and grass overall
-udk comes with a nature scene,take a few foliage meshes from there to give variation to the scene.(careful what plants you use)
-shadows are a bit bloby
-use color grading lut texture
-in udk there is a particle effect of falling leaves.Might want to look at it.
This tutorial might help you a lot(if you havent seen it already)
http://3dmotive.com/series/creating-foliage-for-udk/139/1091