I watched this video about the art style of popular fighting game Guilty Gear Xrd, and something which really interested me was the method that the artists used to get rid of ugly automatic-normal artifacts. They would manually mess with the normals so as to make parts of the model that are round shaded as if they were flat.
The example shown in the video is of a character whose face's normals are grouped on each side, on the chin, and under the eyes so as to make the toon shader look more like a traditional 2D anime.
With modified normals:
Without modified normals:
Notice that this sort of ugly artifacting happens when a toon shader is applied to Suzanne; look at her right eye and nose.
This would be very easily fixable if there were a way to group the normals on the left and right sides of the face.