Excluding rendering there are two major components to a professional line drawing:
1. It has solid draftsmanship (It should look like you can "Hang a hat on it" as one of my mentors (Marshal Vandruff) used to say.
2. It has Rhythm and gesture.
These two qualities of drawing are actually opposites, but professional work particularly in the field of animation has lots of both.
Remember that in art you are not copying reality you are augmenting it!
(Take what you see and turn it up 120%)....