As an amateur writer myself I can just speak of experience.
I think one of the most dangerous pitfalls is obvious fandom (Besides from fan fiction, where that is the point).
A sarcastic mind would suggest, that since Shakespear, none did something original and just tells the same stories over and over again. I don't think that this is the problem.
We as a People feel comfortable with stories, we can relate to and can foresee the ending in some part. For example: The classic love story or a heroes quest.
So where is the line between a professional writer and a spare time scribbler?
A skillful writer knows how to blurry his plans and sources from the reader and keep him guessing, what kind of story he is writing. The reader is pushed to the point, where he wants to read further just to know if the Story will end in the way, he expects and hopes. The Motivation to read becomes even more intrinsic.
But since I am an amateur myself, maybe I am not the most qualified person to answer this question. Who knows?