I know you already acknowledged that you didn't render the lights and darks further. My question is, why not? You have a fairly well-constructed Nicoli. You could easily improve your drawing orders of magnitude by simply making the darks darker. Even just making the pupils their respective deep black could add much more character. Other than that. Your eyes look like they are looking in different directions (I believe the left pupil is slightly offset to the upper left). I believe the shape/outline of your shadows could be paid more attention, especially those under the jawline and in the ear. And your shadows are nearly as light as your half-tones. Remember, the lightest darks are darker than the darkest lights. And there are many parts of your drawing that weren't shaded at all. The only parts of your drawing that should be absolute whites should be your highlights (e.g. It looks like your whole forehead is a highlight). Everything else should be even just slightly shaded so that your drawing doesn't look blown-out. I believe you have much untaped potential in this one! You should come back to it some day :)