I just looked at it again, and I've realised that it's probably the slight angle of the eyes that makes them interesting. Gives some 3-dimensionality to the "face". I'll have to remember that "trick" if I ever do something similar
I'm thinking you could've used a darker background to make the piece have a more ominous feeling to it. Like he's come from the depths of battle ready to kill some more. Other than that, I like the style.