Also, try to vary the buildings some, they are all the same shape, same height, and the same color. You could also clutter up the background with some more buildings to make it look more like a city, rather than strange uniformed pillars out in the middle of some cloud.
I might also suggest making the buildings bigger (not taller), they look too small to be of any practical utility. You can barely fit one room per floor, and half of it needs to be some way of getting from the ground to the top (like a ladder). I'm guessing where the texture repeats is where each floor level is supposed to be, if so, then those are incredibly tiny occupants.
How did she get way up there? There are no steps and no doors to an elevator.
As far as composition, you could also zoom in on the character and put her in the right 1/3rd of the scene (look up the rule of thirds). The twist in the camera is slightly disorienting, I would try making the horizon level. Where is the light coming from? There are no shadows anywhere, and the lack of shading makes the scene look very flat. (I'm guessing the light is directly behind the camera.) I suggest adding 4 lights around the scene that are heavily tinted blue (no shadows) and reduce the main "sun" light intensity.
As a test render it's great, but I don't think it's a desktop yet. Keep working on it, it'll improve.