Hey fellow SSG Niner!
You make me feel so guilty of not drawing. >< Keep it up! (Or don't and make me feel less guilty )
The last picture is my favorite. Very stylish.
Have you learned perspective and vanishing points yet? I find them useful in planning and laying out the environment. It also conveys depth easier. I find that the depth in your environments aren't very convincing.
Beware of your lines. Line quality is extremely important. It isn't just about the correct placement of the lines but also how the lines themself compose the image. This is especially true when it comes to shading. I think shading is all about the subtlies of line quality. It's also something I need to work on.
Hope that helps.