The challenge of drawing and animating skeletons is their lack of mass. There is a lot of negative space between the bones and the bones themselves are thin and thus the artist is presented with the challenge of how to give enough detail to convey the idea but not over complicate and clutter the design.
The next challenge is posing. Due to the uniform colour of a bare bone skeleton. You are often forced to give them some form of clothing to contrast the white bones, or darken the back limbs allowing you to cross the arms across the body. Though with this "nude" design of mine it influenced the movements of the animation, forcing a more open stance to avoid crossing bones over bones.