the trees should be a bit darker, tint them a bit black, you should also try to use more green trees, not just one... put some space between them and add some shrubs and mushrooms between them. try to put a mountain in the background. put a sky and a fog, and add more graves(found in northrend and lordaeron fall tilesets). Also that small water plan in the second screenshot looks bad. the crypt roofs are also bad looking.
also add some short grass and a bit of dirt along with the grass to provide more tile diversification.
But they mainly lack a main object, which could be a unit or a doodad, illuminated by some light source... basically it's the first thing you look at when seeing a screenshot.