Nice typography Cyr, 9.7/10.
I don't know what other people's reasons are for their ratings, but I will try to tell you a few of the things that are bothering me about it. The render, looking at the original, is not smoothly cut. Additionally the lightning in some areas looks very unnatural; toward the point where it passes behind the cape it is very jagged, even boxxy.
I'm not a fan of the + signs in the top portion, I would cut back on them a lot, or even better get rid of them. Blending could also use some work; the cape especially demonstrates this where it should be fading into the background at the right, but instead of fading it looks almost cut off at that transition.
Also, the depth of the sig could stand some improving (I know depth is sometimes the hardest thing to achieve in sigs but it's worth the effort). The biggest problem I see is the left side, especially the big lighting bolt that should be 'behind' him; that general area needs work on blending, burning/dodging, sharpening/bluring, etc.
Last thing is the background you chose looks a bit out of place to me; it could just be the lack of depth, but I'm not liking the (trees?) in the background. Might try making that region a simple color gradient or something along those lines.
Also a side note: don't be afraid to get rid of a render's original effects that can be easily removed without ruining the render and substitute higher quality effects (in example, more natural lightning) that fit with the piece. Be careful if and when you do this though; you must replace every area where you removed effects with other, fitting effects or you run the risk of ruining lighting, etc.
I understand if this may be a lot to take in, but if you want to improve you need to look at criticism patiently and objectively and try to work on each area that can stand improving, this not only improves your piece but helps you learn techniques that will benefit your graphic art skills and teach you what works and what doesn't. Also, look at LOTS of tutorials, especially those that are related to issues you are having.