the thumb is pointed too far forward(towards the camera) making it look massive compared to the other fingers, the back hand should connect with the hip instead of on his tail, the mouth is opened just a tad too much, the ambient lighting needs to be turned up because black teeth are not pleasant to look at, and sonic's knees are glowing 
other than those (and the lack of a skin shader, but no one wants to give those away for free) it looks pretty good!

other than those (and the lack of a skin shader, but no one wants to give those away for free) it looks pretty good!