For me, its hands. Hands are complicated things, especially in art. They are like independent bodies; just imagine the fingers and thumb as a head, two arms and two legs. They can be difficult to draw properly and can showcase the technical skills of the artist. In general, some anime-esque art styles do not always adhere to the natural laws of physics and anatomy, but hands are a fairly universal symbol. They are all essentially drawn with the same formula but can vary greatly from highly stylized to highly detailed. We've all seen cels with badly drawn hands, especially in long running TV series. So, it can be tricky to find well drawn hands.
A lot of my cels feature hands prominently. If I see two cels of the same character and one has a hand showing, it will probably interest me more.
Here are a bunch of random examples from my gallery. Each hand implies a different emotion and feel - whether it's intensity, elegance, rage, sorrow, determination - hands can say a lot.
The stronger emotions the better 
) 
