What kills faster than anything else? Yes, despite my prejudices, even fast than not being character driven?
Breaking internal integrity.
Fantasy by nature is a "What if?" question. That "What if?" is what deviates the book from reality. "What if?" may be "What if there are dragons?" "What if the world was flat and you fell off?" Anything, really. The kicker is that the author must set up limits to their worlds. They set the rules at the beginning of the book. Now, these rules may be "seemingly broken" by the author if they wish the book to evolve past the rules, but they must give a proper set of build ups and cues.
However, sometimes an author will break their own internal integrity. This means they break their own rules. A book thrives on us believing in the alternate reality. If our trust is broken, the story dies. When one part of the rules are made up or broken partway through the book--"Wait. Despite the fact I set up that the waterfalls at the edges of the world lead to alternate realities, really they just form a circle that brings you back to earth." Say wha'?
The reader feels betrayed, tricked, angry.
What books have broken their internal integrity and turned you off them?