shading. Anyway, I hope my comment didn't come off as insulting. Just meant it as a critique.

The arms and face look great. I say the butt looks like a flat plane just because of the sudden transition from light to dark; makes it look like a corner, since it isn't as gradual/soft as the shading used from the waist up. Less noticeable with line art, which would've made it seem almost like cel

Looks mostly good, though the lack of lines framing your style of shading results in certain areas (like the bottom of her butt cheeks) looking like flat planes instead of round shapes/forms.