Wha, a new avatar? Oh, but the realism of your current one is sooo niiicee. I don't think that new one would serve you or your mad skillz justice.
And actually, I disagree on the need for contrast. It just seems like there's no contrast because there's no shading, but each shade in itself is different enough from one another, methinks.
The AA to black doesn't really befit this forum's color, either. And the hair...! I'm all for simplicity, but then there's just plainness!
I don't have boobies so I wouldn't really know, but it looks like the fingers are too high, so the wrist would have to be bent really awkwardly for them to reach the other arm where they do...but like I said, I prolly wouldn't know.
In the greyscale, the big dude's legs...the thighs seem really...well...eh. I mean, they like, totally just like, bend and are all lumpy and stuff. I dunno, it just looks really weird to me. Is the rightmost image unfinished? It looks like it is.
In the last one, maybe the skintones could be reversed, since the left seems a bit color-heavy and the right a bit dull. And in the blue-haired one, that chunk of hair right above the ear looks strange because of the AA to the skin, like it's plastered to the face or something...actually, it looks like that all along the blue-haired one's hairline.
Hmmm...I think that's it!