Very nice, I like it. It reminded me of some good games like Canabalt and Playman ER. Even the black floor doesn`t look weird.
There are some things, though, that do look weird. First of all, I think if you make something in pixel art style you shouldn`t only add it (unless it is kind of an easter egg), you should make everything in this way. I`m talking about the sun, of course. Also the clouds seem to be off since their pixel size doesn`t match the buildings` one. They are also blurred, I see. I`m not sure it is the best decision to make them not-eye-burning.
Then, some logical aspects: the crack-holes on buildings (especially in the one right to the sun and the most right ones) have the background colour like there are no walls behind. The fence (left) is OK, the building in the middle is also alright as it seems to be totally destroyed. Use another colour or add the destroyed walls (of another colour, again) behind the crack.
Then, I guess you drew all the layers separately and just connected them in a gr. redactor like Photoshop. The problem is that sizes of the pixels don`t match here, too. It is seen well on the wires. may be you did it specially (or you just don`t care), I don`t know, but I think the picture would look more harmoniously with entire scale of the pixels.
So, now it is a good background, but it has lots of things to improve and make it better.
P.S. Sorry for my English if it`s too bad (I feel it is).