Sprite critique time!
Atyroki - The main "problem" that I have with your sprite is the head, which is way too small. Proportional to the original drawing, it's correct, but the whole sprite looks... shrunk? Also, I'd like to see a little more color variation in the shiny sprite. The back sprites look really good, though; the shiny color difference is a lot more noticeable.
Flygon493 - A minor nitpick I have is that the front two legs sticking into the ground seem thicker than all the other limbs. Also, I really dislike the head in general. it seems a little bloated, and generally too big. It's also really hard to see the half-smile on it's face, which adds to the "weird" feeling. I do like your backsprite though; the neon-green color is a tad bright, but that's just my opinion.
Wyverii - I really, really like your front sprite. I can't really see anything that I would change; it's definitely one of my favorites. Your back sprite's hand, however, looks... awkward. It would be a little odd for the creature to be holding it's hand like that, I think. Also, there's a small discrepancy between your front and back sprites: the front sprite's hands have "straight" fingers, while the back sprite hands have fingers in a "fan" shape.
Elevator Music - Another one of my current favorite sprites. The front sprite looks near perfect; I really like the poison sludge as an added touch. The back sprite's "arm band" looks a tad frail, though, as the balls of the ball-and-socket joints are showing (which would make for an incredibly weak joint with limbs easily popped out of socket). Also, I can only see 6 arms fitting on the band...
Nasty Pass - To be honest, I don't like your sprites at all. Your first set's hands lack proper perspective and proportion, and the shiny's colors are an eyesore in my opinion. Your second set's head is waaaaay too big, as you said yourself. The hands also look very small, and lack perspective\proportion (the size of the hands in the front is very similar to the size of the hands in the back).
I'm refraining from commenting on the recoloration, as it doesn't look like it'll be allowed anyway.
Raverist - I think your sprite has potential, but the bug-eyes have got to go. I picture this think smirking and throwing sludge in your face, not gaping with huge eyes. I'd be able to give a better opinion it you went ahead and colored it ;)
Zantimonius - I really like the pose. The purple eyes seem a tad strange, though. Not much else to say really... overall, one of the better sprites.
Lawman - The first thing I though when I saw that sprite was "It's fat" :P The arms are a little thick compared to the hands, which could stand to be bigger. I'm not sure I like the way the body's tilted, either, it seems a little too spider-like.
DougJustDoug - I really like your sprite's hands, which are easily the best ones submitted. However, your sprite's head really kills it for me. It just doesn't look... right. I'm not sure what exactly I don't like... I think it's a combination of the eye color, eye shape, and the ears. I really like your shiny colors though, I've always been a sucker for black shinies ;)
Dragonzrule - Seems decent, color please.
the_artic_one - I think your sprite has potential. The eyes, however, have got to go (see my comment's on Raverist's sprite). Also, it needs shading.
legacy_raider - I'm torn on your sprite. On one hand, I like the pose, and I definitely like the sludge ball. On the other hand, the whole thing looks a little spindly, as the body is too big in proportion to the legs and arms. I definitely think that it has potential, though.
CyzirVisheen - Your sprite is another one of my favorites. If I had to nitpick, I'd say that your drill is a little small. Other than that, it's easily one of the best sprites submitted.
IceBug - Your sprite looks... cartoony, and not in a good way. I do like your pose though. I strongly suggest taking a close look at some of the other sprite submissions for examples of how to color.
Oh, and one last nitpick. The following people's sprites do not have transparency: Nasty Pass, Raverist, Lawman, Dragonzrule, the_artic_one, legacy_raider, and IceBug.