Concept art always seemed so tricky to me... I love it, it looks so simple to start, but then I just get lost along the way... I never use the right brushes... and that's not my only problem, anyway... Your work is brilliant!
I used a ton of photo textures for the mountains, and that combined with accurate colors and proper blending is responsible for the realistic look. And of course, Jaime Jones' legendary cloud brush!
Also, the composition can easily fool you, for example I barely did anything to the right side except pasted one texture and did some blocking in, instead I spent much more time loving the main focal point area. Also the fact that this piece is so dark allowed me to get away with much less painting and texturing, if you look at the top of the left-side cliff you can see how I just mimicked the texture of the lower areas of the cliff with quick brushwork, which was enough to give a convincing look.