Grammatical errors drive me nuts, and once I've spotted them, I can't un-see them.
I also find it deeply annoying when fantasy authors 'create' or 'imagine' a society that is essentially a copy of a real society (usually a medieval one), basically ripping it off and adding just a few details to make it look as if it is original.
I started a series of books a few months ago and the first volume seemed really promising, but in the second book the author had his hero travel to a mysterious country to learn sword-fighting from !!The Best Martial Artists In The World Ever!! (TM). The country turned out to be medieval Japan with the genders reversed, so that the women were the amazing martial artists. The men sort of disappeared from the picture, but I imagine they were busy with housework.
Then the hero got beaten in a sword-fight by an 8-year-old girl, which made me go