Another is not knowing the difference between you're and your. I mean, seriously, this is grade school level ****. I've always seen grammar Nazis as people that complain about run on sentences, lack of punctuation, using the wrong punctuation (for example, when to use colon and semi-colon).
Everyone should read "Eats, Shoots, and Leaves" if you are capable of reading a book, that is.
As an American (US, that is, Canadians are probably much closer to Olde Mum Angleterre in this) it made me realize that proper English punctuation is often superior, although I still prefer US spelling (how many times does the letter "i" occur in the name of the element "aluminum" and what vowel is in the name of the color (a word lacking in a "u") that is a mixture of black and white?).