Piece 1 - "Bottled Self"

This piece was actually a self portrait I did during my digital arts class.
The concept I felt was strong, in that it conveys how people tend to categorize and separate different parts of themselves that make them up as a whole person. And even though the rendering of flesh is good in most places like the feet and hands, it's extremely disproportionate. The head seems a bit too large and the body (arms and torso) significantly too small. The legs that are detached, although relatively realistic in rendering as well as an intriguing part of the piece, is also a bit too large for the body.
The character in the space, although we can tell that the character is in some forgotten and grungy location, doesn't seem to fit the space in size and color. The background and foreground don't match in color temperature and lighting making the foreground pop out much more than it should.
I personally am not a fan of mixing photography and drawing so that also seems very off to me even though this is obviously a surreal image. I think the images of the artist in the bottles are intriguing, but the photo realistic bottles don't match the scene and tend to pull more attention than they should. Also the storage of bottles in the background should be a bit smaller considering the distance.
Back to the figure, now that I look at the face, the self portrait isn't very visible, mostly because I made the mouth and chin a bit too small (or the nose too big...what can I say, I love drawing noses!).
All in all, great concept (that's why I love this piece so much), but when I redo it, I think I'll execute a bit differently.
Piece 2 - "Snow Adventurer"
This is also one of my favorite images. I think it's because I normally don't add too many accessories with my character drawings and this one is just filled with accessories! I personally like the slight disproportion of the back pack (which seems a bit larger than it should be) but it's a good exaggeration on how much things an adventurer needs on an adventure.
The body itself is decent except for a few proportion issues. 1) The figure's head is a bit large. I more prone to drawing larger heads (AKA shorter people, AKA younger proportions than full adult). 2) The legs aren't proportionate to how they should be if they were foreshortened. Same for the arms as well but it isn't as obvious as it is in the legs. 3) I would probably also make her hips a bit wider, however not all women have the hourglass shape so that just might be a personal preference for this particular character.
I think my largest gripe is about the legs. The leg on the left seems less like it's forward in space and more like it's scrunched up. The other leg isn't really much better with the very short thigh and much longer calf.
Piece 3 - "Wendy Michaels AKA 
Ahhh Wendy. This is a fanart of one of my characters from a role playing game. She was actually a super hero in the Marvel Universe, although she's older in this image than the original story. Well, enough reminiscing...on to the critique!
I love the rendering for this image. The color is nicely done with very crisp and obvious shading and highlights. It is one of my better marker colored pieces. And even though the proportions of the face are not to standard, her face is my particular cartoon/comic style, where there are slightly larger facial features (lips, eyes, and mouth). Her hair actually seems to fit on her scalp pretty well and doesn't appear too larger for her head, which can sometimes happen in a comic/manga style art. If I were to change any proportion on the face, I would probably make the head a bit smaller to accommodate the smaller neck (or make the neck a bit thicker).
Her body proportions however, are quite off. I think really the most distracting are her hands. They're definitely a bit too large (or rather her fingers are) than they need to be. I also feel that her chest seems a bit off, and should be a bit smaller/shorter where her torso should be longer (Asian proportion are generally shorter legs with a longer torso) and her hips should be a more gradual slope.
Her arms aren't too bad, considering that she's suppose to be the super strength mutant to where I think there should be a bit more muscle definition.
Piece 4 - "Insert_Witty_Username Fanart"
This wasn't necessarily fanart. I join a few avatar sites and enjoy drawing peoples' avatars for in-game cash. It's a fun hobby to practice and I enjoy the unique outfits that people create. =)
I particularly like this one, since, I believe this was the first time I used SketchBook Pro (awesome program for digital sketching btw) to do a full body illustration. I also like the coloring I did for the skin (I like the look of blushes on the joints).
As for proportions, I didn't base this off of a figure (most of my art isn't) to sketch from, so I tend to always make the head larger than it should be. However for the nature of this style of art manga/comic, it's a very common feature. I think it looks a bit unsettling though, compared to other pieces (like Wendy Michaels and Snow Adventurer), since the figure is VERY long-limbed. It almost looks ridiculous on how small the hands and size of the arms are as well. Also the REALLY long legs makes the character look really tall when the head size says otherwise. I think I would also make the waist a bit thicker, it looks a bit too uncanny for such a lean looking figure to have very wide hips that suggest hour-glass figure.
Piece 5 - "Voltaire Bust II"
During my undergrad, a lot of still-life/figure drawing was had on the busts in the art department. And none so popular than Voltaire. (I loved that Voltaire bust to death! He was so much fun to draw!)
This piece critique will be a bit different since the image is only of the head (it's actually a section close up of a larger image); it's also in medium I'm not very comfortable with...charcoal. Anyway, let's get straight to it then!
So I have this fascination I find with noses. I always tend to draw them a bit larger than they need to be, even though Voltaire's nose was pretty big. I think the rendering of the charcoal was relatively decent and you get a good idea of the volume and the depth of places like the eyes and cheek bone. However, he looks a bit cartoonified. I think it actually has to do with the proportions. All of the facial features are slightly too large for the face. The jaw line is one of the few things too small and is actually a bit more narrow than it should be. The forehead is a bit large and makes the head seem huge with a very narrow chin (like Mandark from Dexter's Laboratory).