You'll never guess

This is a nicely-composed and shot portrait, but as Kirol says I think the focus is a little off. I bet I know what happened: you used the autofocus in your camera, probably set to the evaluative mode, and it focused on the parts of your model closest to the camera: her hair and tip of her nose. You can get away with this with a shallower aperture, but not at f/1.8; either stopping down a bit or adjusting the focus so the eyes are sharp would result in a more pleasing photo, I think. Apart from that it's great; the only thing I'd change is airbrushing out the small white-grey spot on her collar, on the middle left of the image.