Awesome! You're doing GREAT for you first try at this. I know you've gotten rid of all the color problems by turning it B&W, but I really think it should be that way. There are some stuff here and there that you should work on, but they are already mentioned in this thread. Although, the clouds above the city doesn't look bad, they are just a bit too contrasting. Just make them "thinner". Also, I don't think that you should add some kind of nebula. It would ruin the cool minimalism at the top. Keep it this way, but you could work a little on the starfield to make it look perfect.
I'd love to see a bigger version. ^^