Size and Shape-- Often people chosse avatars that are to small and they get inlarged an ugly looking bit avatar. When that happenes its hard to tell what the avatar should be and takes away from the whole avatar. If that avatar is animated thats even worse.

Animated or not-- Alot of the time the deciding factor of this is weither the site soupports it or not. Often times it does. If the animation of poorly the whole avatar can be messed up. A lot of the time just a simple still image can say alot more then a ugly moving one.

Representation-- Most people dont notice but when they pick an avatar they pick out something they like. Try and pick one that tells people something about you, unless that is you dont want people to know anything about who you are.

hope that helped out about and sorry about not having any exmple pictures i hate looking thourgh google for examples and stuff..