• Names including a lot of consonants and apostrophes, eg Zz’gs’trymz’n.
• High-falutin’ bollocks like “The dragonmage Zz’gs’trymz’n strode forth, his sky-forged sword Hg’grin’dyr the Ever Silent at his side, to join battle with the foe”.
• Historical fiction where the author gets details wrong – looking at you, tight-laced corsets (most weren’t) and women running down the street not wearing hats and gloves.
• Authors making up swear words. (You've imported names and offensive gestures from our English-speaking world: why make up some new word and use it exclusively when we have many, many excellent words that would do the job, especially as context indicates that the well-known and often-used “fuck” is what you mean?)
• Fantasy books that are not internally consistent. If you mention that magic can’t, for example, create fire, magic should not create fire later in the book/series.
• Books set in English towns (I’m sure this applies to other countries) that are obviously written by an American whose closest contact with said town is a cursory glance at Google Earth.
• Books set in England (again, I’m sure this applies to other countries) that are obviously written by an American and use American vocabulary – it’s a car park not a parking lot, a pavement not a sidewalk.
• Stilted vocabulary. This is supposed to be a modern 16 year old girl speaking to her boyfriend: "The landscape had changed. The paths I remembered were no longer there. The hills and valleys almost seemed to have been reordered." "I became excited and he looked more than a little sad... Of course, I didn't heed his warnings".
• Hard science fiction that's just bang-bang-shoot-'em-up set in space with a little spaceship maintenance for the people who like tinkering under the hood of the car.