Nothing irks me quite like chipped nail polish, yellow and stained teeth and poor grammar.
Lastly but most offensive of all, poor grammar. While I am totally forgiving and guilty of one-time offenses of poor grammar, I just wish people paid more attention to their usage of words. Get your "your" and "you're"'s right. Know the difference between "then" and "than". Anyone who went through 5th grade English class will know better. If you don't know better then it's probably time to pick up a book.