I like it a ton--it's one of my favorite CC games. I think the art looks great and lighting is where you should head next. Two things that can make your graphics look even nicer:
-Outdoor shadows are typically blue. Look at a photo of a sidewalk, or isolate it and use an ink dropper in photoshop to select that shadow color--you'll pick dark blue (instead of dark gray). This is most apparent in snow. Any daytime snowy photo will have blue all over it. In all cases, these are reflections of the sky's color.
-Another trick to make believable shading without it overlapping and doubling (shadows combine in real life) is to import them at 100% opacity and put them all on the same layer. Then set that layer's opacity down to whatever you want. This helps the believability nicely without a ton of effort.
In addition to those, subtle gradients and such over top of the screen for lighting can make a big difference, too. Give some of it a shot--you've already got some of the best art around.