I currently most like the hand-drawn/anime style of drawing right now, so that's what I'm working on mostly in regards to faces. I've learned proportions are the most important things when doing faces and I feel like I finally understand how to do at least one character properly.
Tonight, while working on a sheepish expression, I accidentally made it look sultry. Then I figured out why and fixed it: the top of the eyes were more flat than curved upwards. It was a simple fix, but the change was incredible.
The face on the right is the first one I drew, the fixed one. The one on the I quickly drew so I could show a side by side comparison. Of course it's not proportioned correctly like the other, but I think the point still comes across.