Ok, so you're definantly more art oriented than you are design oriented. The main thing you have to remember is to simplify things. With this piece it's hard to tell what the point of the sig is. There is more emphasis on Marcus and the spartan than there is on the xbox. There are to many colors and to many things going on at once. It might look better if these were split into 3 different images.
Many members have been on a break and we could always use help with sigs. I want to see you try some requests. I don't want you to post any of them without my say so first though. Try to find some requests that aren't filled and do the requests. Post the image you make and a link to the request. If it is deemed good enough I'll let you post it. (This doesn't mean you're on the team yet since it's not up to me, but you're so active you might as well help us out. Doing requests might even help you grow with your art).
Ok. i have noticed i do abit better with requested art than i do with coming up with it on my own so ill see what i can do and post it here