When I first got interested in Sig making, I searched out for different graphics communities and had them critique my work. I greatly increased my skill through a small amount of time just because of the basics that most self-taugh photoshoppers don't pick up on their own. Such as a necessary light source, focus, depth, and text.
Notice the lack of a light source in your sig. It appears that (gah it escapes me what the Halo character is called) the light is coming from his right and from behind, and yet there is nothing there that shows this. Use a photo flare or something of that nature to produce the necessary light. Also, your text does not fit in with the sig. Maybe a more grunge-like text. Also, try blurring the background and sharpening the character to define the focus point.
Don't take any offense, just some pointers
