This applies to basically any story in any genre, but I hate it, HATE IT, when writers write accents phonetically into a story.
As soon as I see something like: "Oi Whot! Aie dere Guvnur'" I go into a kind of killing rage.
Books have gone hurtling across the room, pages flapping, for this kind of offense.
This is probably the biggest deal breaker for me. Tenses, themes, settings, politics, random sex, nonrandom sex, drugs, royalty, method of travel, aliens, robots, green dancing girls, silly heros that always win the day, twists, detectives stories, cute aliens, ugly aliens, using alien life as a metaphor for racism in America, aliens that look like people, robots that look like people, military themes, violence, utopia, old themes written by new writers, new themes written by old writers, outdated or dated genres, alternate histories and silly parodies are all fine with me, as long as they are done well, but the arbitrary definition of well that I set for the fiction I like.