Seeing this sprite earlier today, I couldn't help but try to give some critque or ideas of how to improve.

First of all, here's my edit of the shading to help explain things; puu.sh/hckvS.png

-You used a lot of colours to shade, which didn't give the shell much depth as the colours just blurred together with many of them being too similiar to eachother. Try to limit the amount of colours you use, starting off with trying to shade using only 3 shades (a light, a dark and a middle between those two) and then possibly add another shade to perhaps show the shell has a shine or something.

-The lines within the sprite don't match, as some are coloured while others are not. Try to stick with either having an all dark outline around each setion, or only around the edges. Maybe try no outlines at all if you're feeling adventurous!

-the light source is undefined, leading to some confusing choices with how some sections are shaded. a good tip is to put a light source somewhere around the sprite, then shade according to where that light source was set. Doesn't have to be fancy, I myself just use a small yellow circle to help guide me on where the lighting should be.

Overall though, you do have a good shape for this sprite (although some lines could use some smoothing out) So just keep practicing! I'm sure you'll get better in no time. Feel free to zoom in and take a look at how I've shaded mine for comparison!

I'd say that there's too many black outlines within the sprite, it makes it look pretty crude and rough, espcially with how the outline on our left is shaded unlike the rest.

There's also quite abit afew cases of where the shading is just one line of pixels next to the edge. I'd recommend trying to reduce or even increase these areas to either give it some Anti-Aliasing or more detailed shading.

Of course, everyone has their own unique style! I guess I just converted the sprite into more of my style to show some ideas that can be useful. Glad it was helpful!

And about the teeth, I think it's just that small details like having missing teeth is much harder to convert into pixel art due to the limited canvas you'd be working with. On my version I decided that instead of showing teeth on the top and bottom of the mouth, just having a row on the top with gaps shows that he has missing teeth but it's not needing to use so much space to show the whole set of teeth.

Hope that makes sense, sometimes I'm not good at explaining these things ^^;

I saw this sprite the other day and thought of some criticisms...then forgot to post them, oops.

Anyways, I played around with this sprite while I was bored as I liked what you had so far: puu.sh/gUcBj.png

And the things I noted;

-the hair would stand to work better as a large clump together with shading to indicate that there are seperate strands within the sprite

-the white looked too flat as the shades were not showing up well. infact the shading overall should be more defined.

-there were a lot of pixels that were just straight lines, especially in the arms.

-why does the mouth have an outline, but the rest of the sprite doesn't? Either give everything an outline, or only the outside.

-You could potentially remove some colours as they could be replaced by others, such as using the darker skin shades for his tongue and the red of his outfit for the mouth

-some AA could be used in places such as the hat or  body

That's all I can think of for now, hope this helps!