Generally, characterization is my biggest concern. I can even read a fic of a pairing I'm not crazy about and appreciate if the characters still feel like themselves to me. But when you start changing them into other people in order to write a pairing? Then I have a problem.

That said? To avoid this, I usually don't read pairings I don't like unless I know the author is generally good at stuff like that. My policy with ships tends to be: if you don't like it then don't read it. It's that simple. No one is shoving your head against the computer screen with a gun and forcing you to read.