try playing with more that one light source. that robot for example would pop alot more if he had some sort of intresting light source landing on his back side, even if it was real subltle it would add a lot.
great work! the only criticism I have is that in the first drawing you did with your tablet, the eyes aren't in line with the rest of the face, they're too far to the left. Besides that I've got nothing. I love your robot, he's awesome ^_^