You made a few mistakes. First of all, your brush is either too sharp or you didn't add enough strokes. You can see the pixel lines on your black outlines. If you remove the white glow it might look better. If you use a softer brush focusing more on using tablet pressure and size/opacity control, you could've made the contours more circular. Instead of just using one line to go all the way around, us smaller but more strokes and then trim any excess. Make sure you do this in transparent layers of black, color, then whatever you want under, but you probably knew that.
Secondly, the teeth aren't very symmetrical. the tooth on the left side of the picture is fanning out than the other, but unless you were going for an uneven or unequal style that's not going to work. The eye sockets are different sizes and I would recommend adding a little skull on the left side of his (her?) left socket.
Your shading is good, better than I can do realistically. It looks like you have a hard time showing 3-D shading though.
I would recommend a reference picture, also.
Happy drawing!