This images is one of my favorites. I like the perspective shift on the image (especially in the clouds in the upper left). It gives the clouds the feeling of motion. Terragen includes zoom controls for the camera which produce accurate distortions in the image as the zoom level is changed.
One of the other things that I like a lot about this image is the emphasis of the detail of the rock. The many ridges and high level of detail/variation in the rock surface really make this look ... well real.