stephen,
its a great composition,
in my opinion, the nudity distracts from intented emotion of the piece… her clothes etc indicate her intelligence (and her capacity for experiencing melancholy)… theres a certain schoolgirl look about her which doesn’t make me really empathise with her (hey maybe there are alot of schoolgirls who might disagree) but to me it suggests shes more “sad” than melancholic… it puts a visible limit on the depth and maturity of her emotion and makes me feel uncomfortable with empathising with her… I’d personally go for either more clothes, or stylising it a bit (a leg band perhaps… a symbolic tattoo…) to indicate to the viewer that they shouldn’t be placing any preconceptions as to her scenario (prostitute?) because she obviously is from another time/place…
you’re right about showing the face, however using the hairband is imho a bad device for achieving this… i doubt she would have the drive to pin her hair back like that if she is melancholic… Assuming she has recently lost her arm, there would probably have been some kind of struggle which would have loosened her hair from its tie, and I doubt she would have pinned it up after losing her arm (generally speaking, tying my hair up is the last thing i’m thinking about when i lose my arm… especially when its on a side of my head which is awkward to pin back using my one good arm…) I’ve put on half/half so you can see the face subtly enough to just get an idea of what she looks like, but no so much that it seems forced…
this is totally subjective here, but i’d like to see her focus her blues on something visible (so I can see what she is thinking about). As it stands, her blues are hidden in her mind… her staring at something in her hand gives me the feeling that there is another action about to take place… its not empty despair, but despair which will lead to some further action… (personally i think she’s thinking about wedging that screwdriver into her CPU and ending it all… but thats just me.
) This might ruin your beautifully painted hand, so i didn’t really want to suggest it… but there… i said it! :)… I’d think a little about the scenario too… has she been banished? where is she? an alley? a basement? what is she holding in her hand and why? who did this… what is she thinking? what is she feeling? (eg. ‘I hate myself for the decisions that led to me being in this situation’ or ‘I resign myself to having no arm and must now consider if it is worth living or dying?’ or ‘Boy i think i really like vanilla?’)
The stump of the arm looks a little strange in my opinion also, as I don’t believe a clean break of an arm (robotic or otherwise) would occur that close to the neck… i’ve stretched it out a little in my modified version here to try and fix this, while at the same time getting rid of the ‘roundness’ of the hole
the rolls in the stomach aren’t as convincing as the could be (but you probably knew that) the right breast looked a little flat… ie there wasn’t any shading or shadow as there should be (as indicated by the other breast and the shadow on the wall) the bra sits flat against the breast, which it wouldn’t if it was loose like that
i’ve included an image here with some (really messy and rushed) modifications

those are my thoughts… very picky, and very subjective (but thats what you came here for right?) its a really brilliant piece, but could be made even better by further clarifying the context in which it sits…
cheers
lachlan