Instead of picking this piece apart I will go and tell you what to work on for your next piece. At this stage in your process you need volume. The more you do, the more you'll refine your style and start to get a feel for the tools and the rules.
First is composition, it is one of the big three factors that make a sig amazing. Composition is things like focal and effect placement, as well as things like flow. Next is depth, depth give your signature a 3D look and makes it look more interesting to the eye. Finally is lighting/color - they're grouped together because color is directly effected by lighting. Think of your piece as a 3D scene and you are the camera man. Where are the shadows going to fall? Paint them in. How will the light hit? Makes highlights accordingly. How will this all effect the colors of the piece?