The shape of the eyebrows is VERY important. It can change the look of the skin in a dramatic way, and the ideal shape and height change for almost every skin. As the eyebrows are not under the "Shape" menu, this is often overlooked.
Another little bit that matters, apart from the obvious lips, nose, lip cleft yadda yadda, is the "eyebags" slider and its close companions (can't remember the exact names). It mostly affects the expression, in a permanent way. So you can be moody
, or always surprised
, or permanently bored
.
So, hmm. Buying a skin: it's all about the shape.