Glad to see another talented artist around LuxA! I meant to comment on your previous B&W mesmer portrait, but I've been occupied this past week with only a minute or two at a time to sit at the computer -not nearly enough time to write up the kind of critique your work deserves.
One thing that jumped out at me on both paintings is the hair. Overall it's very well done. The wisps, however, are blocky due to the fact that we can clearly see the brush you were using there. Just a tiny use of the smudge tool at the very end (the only time I'd suggest using that thing) to pull the strands to a finer, softer point would make a world of difference.
I'm sorry. I don't usually say anything negative right off the bat, but that's such a minor point which I think would really improve the final feel of the painting.
Now let me say some nice things. The soft blue background really works as a nice contrast to her skin and lips and helps to set the mood of the painting. You also do a wonderful job of the hair. You're able to highlight and shadow the hair to bring out not only the individual strands, but also the body and shape of the hair as a whole. In addition, you do it without causing the two to fight.
The subtle detail of the zipper, her lips, and her eyelashes are very nice. They show that even if some aspects of the portrait are lacking in detail, you definitely have the eye for such things and the talent to put details in.
Of course, there's always room for improvement. Two main problems stand out to me. The first is that the shading is done with white and black. Shadows are black; highlights are white. I feel like this is something I complain about nearly every single time I look at someone's paintings (my own included at times...

), but it really makes a world of difference. There is very rarely any time in real life when a shadow is black or when a highlight (barring the extreme tiny point highlight) is white. I never really understood how important the avoidance of black and white in shadowing was until I read this. I know it's a lot of reading, but it's extremely helpful.
Following from the use of black and white as your means of shading comes the fact that color-wise, I think her face is very flat. The biggest point where this stands out is in her nose. Out of the entire face, the nose should be one of the most red areas. Even if it's not red, it should be more red than anything else (except for maybe the cheeks). In this painting, however, the nose is almost grey. A nose is defined by highlight and shadow more than any other part of the face, and thus using black and white as shading implements will end up greying it out.
Skin is really hard to get right though, and I'm still far from it myself. I've found these two links to be quite helpful though:
http://forums.sijun.com/viewtopic.php?p=304171
http://forums.sijun.com/viewtopic.php?p=304170
The other problem that jumps out at me is that while her face is obviously supposed to be angled to the side a bit, it still feels like it's flat facing us (the nose wouldn't be bent or anything if the face were tilted, but if it were flat on it would certainly be crooked to the side). I think there are a couple of reasons for this.
For one, although the eyes vary in size to show depth they are on a flat line. You can draw one straight line and it will hit through each corner of both eyes. While it seems like our eyes are on a flat surface on the front of our heads, they're actually not. Our eyes are actually sitting on a plane that is slightly angled backwards, as the inner corner of our eyes is farther forward than the outer corner of our eyes. The result is that when our heads turn, you'll need a curved line rather than a straight one to line up the eyes. If the eyes fall on a straight line, our minds will read that as facing directly towards us regardless of any size difference between the two eyes.
The other reason I believe the face feels strait on rather than turned slightly as it's supposed to be, is the way the light falls on her cheek. While the shadowed cheek would suggest that her head is turned (how else would that whole side of her face fall into shadow?), the lighted cheek has its brightest point very far on the side. The closer a highlight is to the edge of an object, the more it will look like it's something flat facing us. By bringing in that brightest part of the cheek more towards the center, even slightly, you'd give the feel that her face is rounder or turned.
I do agree with Sidra and PieXags that there's a soft bluriness to a lot of the skin. However, it doesn't bother me nearly as much as these other things. I'd be willing to forgive the softness of the skin as a stylistic effect, where I have a very hard time overlooking the rest.
Edit: "Shadows are white; highlights are black." ...what am I, dislexic? >.<