Too often, I hear the advice imparted to new writers is along the lines of "don't write a Mary Sue" or "make sure your grammar is consistent". Although I am rather opposed to teaching writing in such a way, I am also interested in what sorts of rules people believe are never broken in good writing.
Along the lines of my last topic about the biggest problem in writing, this time I'd like to know what rules you think stories should follow. I'm not talking about what makes a good story, mind you, because a piece of writing could follow any rules you throw at it and might still be garbage. For the sake of this hypothesis, assume that the grammar has no problems, as I am focussing more on the higher-level content of the story rather than the technical ability of the writer.