First thing I see is the lack of contrast. It's verry flat and grey-ish, you might want to pump up the saturation and darkness of the dark portion of your palette. Make the squint test with your eyes, and you will get the feeling of your picture. Nothing stands out immediately, and you probably want the player to focus on certain elements more than other or parts of the picture to stand out even when the player isn't looking directly at it.
I made a quick n' dirty edit, just paint bucketed the darker colours with themselves in multiply mode, the darker, the more time I clicked (10% at each time).
Other than that, it's a pretty good start!