Well, I have to say that this is a really nice piece. The feature I just couldn't get over had to be the reflections and highlights. All I have to say is WOW, well done! The highlights are consistent with the light source, give it the 3D feel (along with the shadows), and also gives it a nice texture, especially with the helmet. In combination with the rough lineart, the overall composition was quite an eye catcher (almost gave me that comic book feel).
My only gripes are the background, which seems bland and lacks some unity with Chief's olive armor, and the head seems overly detailed compared to the rest of the body. Both only require minor adjustments, like adding some shadowing and highlights to the lower half of the image. For the background, a simple tinge of green could be just enough to get the image a bit more "unified," but personally, I would like to see some more activity as well.
Still, this is a well-executed piece that certainly would make it into my faves any day. The highlighting and reflections around the helmet were especially impressive (I still can't get over how awesome they are), and love the graphic novel feel of the entire image. Well done!