So, I saw this picture on Facebook, and I'm sorry to say but it gave me "Uncanny Valley" shivers:

I've been staring at it trying to figure out what's wrong. I've determined that by far the main thing wrong with it is the eyes. They look dead, and they are staring right into my soul.
It is actually something very simple but very essential: They don't have any shine. Search "Sonic" in Google Images, and you'll notice that every picture of him has that little white shine on the pupil / iris area. I don't know why this is so important, it's very small, but it seems to determine whether or not I find it nightmare-inducing.
To test out my theory, I put little white shines on it in MS Paint, and it helped considerably!

I also tried removing the fang while I was at it, and I gotta say, I think I like it better. I mean, fangs are a cool idea, but I think they make his face less relatable or something.

I don't know, does anyone else think the second one looks more appealing? (despite my rushed MS Paint work)
EDIT: After I posted this, I keep noticing faces, whether anime, 3D animated, or live action -- and the eye shine is almost always there. As I understand it, the Uncanny Valley results from our brain's desire to avoid diseases and such. I might hypothesize that, when I look straight into someone's eyes and there's no shine, my subconscious brain takes this as an indication that there is something very unhealthy about this person that is making his or her eyes not shiny; and so it tells me: No! Stay away!
Now that I think about it, when I'm drawing an eye, I've noticed that there's this one moment when all of a sudden it clicks and suddenly I'm looking at a person's eye instead of shapes. I guess this must be what they mean when they say that "the eyes are the window to the soul."
I could go either way with the fangs... But I'm thoroughly convinced that the eye shine is necessary. (Sorry, I've probably made that clear at this point. I tend to be a bit long-winded, lol...

)