It's the lighting that does it for me; even though that's the sun creating the light it can easily be interpreted as something greater, as if adventures abound where the imagery ends. Bierstadt's landscapes are gorgeous and he captures lighting wonderfully, so the paintings feel both real and mystical at the same time. Looking at the painting I can imagine all kinds of things in addition to the scene that's been depicted; a small town, maybe, situated around the river or a castle on the high rocks to the left.
I have a great love of landscapes, especially mountains and forests with no human or artificial presence, and Bierstadt really does it for me. I can take any one of his paintings and add a fantastical slant to it, but that's usually after I'm done drinking in how gorgeous his work is.