That is one enormous head, isn't it? Regardless, you've got your horizon line aligned with her eyes, there are some really nice things going on with the lighting, and the background does a good job of setting the tone.
I see something that's bugging me about the way the background is done, but I can't place it, so I won't subtract any stars. It would be unfair to knock off points without telling you why. I think it might be something to do with the lighting clashing with the perspective, but I'm not sure.
My main problem with this picture is the size of her head, but then there's her hair. The way it's done, it looks like it's at an angle, falling and landing behind her dress, a foot or two beyond where it should drop. Granted, this is really hard to say for sure from the angle she's at.
Speaking of angles, I just noticed that she's sitting at an angle. Her shoulders are straight, but her dress is diagonal, like there's a ramp in the floor that she's sitting on.
In closing, I'd like to restate how much I like the lighting in the background, even if I can't identify what's bugging be about it. It'll dawn on me next week or something.