This is a very well-dome piece, especially in terms of anatomy and very natural pose. The only thing that bothers me in her body would be her left ankle, which looks a bit too wide or swollen.
What you could definitely improve is the background. It seems too simple and somehow as if it was done in a hurry. The orange wall is too vivid and distracts viewer from the character. The 'bricks' effect looks too much like a default/pre-made effect and in my opinion doesn't make much sense. I know you aimed for a rather abstract background, but I guess that a blank wall with a calm color would work much better here.
I like however the bench/bed that she's sitting on, though it needs more details and the lines in order to match the style of the character.
You did a very good job with the coloring, I especially how you rendered her cheekbones. My suggestion to your coloring technique would be to use more variety in colors and more contrasts in shadows. Don't be afraid to add some blues or purples to the skin, it will surely look more alive and interesting. The contrasts would emphasize the three-dimentional forms, because now everything looks rather flat, especially in the hair part. That's why you also should mind the source of light - once you set your main source at the beginning, it's easier then to control the shadows and create the forms.
I like the details you put on her clothes. The folds on her jacket are really nicely done, however - like I said before - it would be great if you used other colors to enrich the values. I'm also not use if any woman would be eager to wear no bra while being dressed so elegantly, but maybe she's just posing for her lover, so who knows. xD
And lastly, I think that this picture would benefit a lot if her face expression was more specified. It's good if emotions on drawings are exaggerated, because it makes them noticable and believable.
I hope that's helpful! ^^
Your observations are truly relevant and they are really useful for me! I know I still hesitation to use higher contrast and also do not like to develop background! I am at the beginning (I have 5 years since I started drawing seriously) so I really appreciate the advice of others artists! Thanks so much again! Best regards!