One thing that I have found that tends to make landscape renders look fake is the lack of background. You've got a very nice foreground, but it looks like the world ends right behind it. With a terrain such as this I can imagine tons of rolling hills behind it. Also the sky seems as though it is perpendicular to the ground when in reality it should be parallel. Give the sky some perspective w/ clouds becoming smaller the farther they are and maybe some atmospheric moisture.
Nice pic!

Steve Newport