The Mary Sue character reads like a too-perfect version of the author. With no faults, she has no depth or complexity, and therefore no story worth caring about. She drags down the strongest plot, but if you know her characteristics, you can transform her into someone real on the rewrite.
In order to write successfully in a certain genre, you must know the unwritten rules – in short, what the readers in that genre want to see. This is overlooked by countless writers who either jump eagerly into a specific genre without researching it first, or knowingly try to break the rules.