Bad grammar that even makes me want to hit people. I'm dutch so when I'm able to correct the stories of American or British people, I quit, When the writers mother tongue isn't English (or American) then I sometimes stick with the story if there aren't too many errors.
Having too much crazy names or when the main charactergroup has almost similar names. The writer might know who is who, but sometimes I still need to look back into the beginning chapters to understand who was who again.... Same goes for personality's that are the same.....
too much explaining, and especially when it is written between parenthesis. You need to write a story, that includes details....
Ohyeah, and Author notes between the lines that comment on something a character does...... EEEEEW!!!!!