The shading style here seems to be "the closer to an outline, the darker it gets" (and just the opposite for the background). Instead of giving the picture depth, this seems to make it flatter. Try defining a particular direction for a light source; in this instance, a particularly dramatic light source might be simply directly overhead, or overhead and slightly to one side--this will make your angel character stand out and add drama to the composition.

The blue in the background battles with the blue in the jeans for attention; you may want to pick a different color, or make the blue in the background darker, or conversely, make the blue in the jeans brighter.

The lineart for the most part is fine, though thickening the lines in certain areas would probably make it more interesting. The poor guy looks like his head has been chopped off, however. You may want to show more neck and less shirt, and move his hair a little to the right so it seems more connected to the body.

Hope this helps.