One thing you need for realism that no one has mentioned is Scale. Your scene looks small because you probably used the terrain at almost the same size they were created. One trick I use is to make all my terrain very large. I try to make the camera the same proportionate size as a human head in the scene. This way the atmospheric perspective looks more realistic and you get a better feeling of size and depth. Also if you move your camera further into the scene and widen your focal length you will get a feeling that you are in the scene and not looking at it from a distance. Another tip to remember for depth is "Kai's Law", which says that you should place an object of high detail close to the camera to make the background seem less detailed. Besides that, an old standard is "The Rule of Thirds". Divide your frame into three parts; foreground, middleground and background. This also helps make your composition more interesting, right now you have a middle and background, but no foreground. Keep at it though, you'll eventually get it.