It's not POV, or style, or plotting or grammar or sentence structure.
Sure, those things help, but they're not the ONE RULE.
Character arcs are background singers and bass guitars.
Tone is best supporting actor with only a line or two in our main feature.
Books on dialogue and scene building are assistant managers and vice presidents, all of them bowing and cowing to the ONE RULE that is too often unmentioned.
Forget about spell-checker. That's a tool, not a rule.
Descriptives. Character depth. Proof-reading and revision. They lay at her feet.
She's felt, the ONE RULE, but not heard.
So what's the ONE RULE? What are those two magic words?
Now why is that so hard?