Pretty much anyone who is very serious about being a 'grammar nazi' is annoying as shit. They usually aren't very bright themselves, so they take this one thing and nitpick it to death like that's making up for it. It's especially horrible when they are the kind who want to use big words that they don't use in the right context, or use words they don't really need to so as to sound even smarter. Someone I know once said the phrase "it will send you a digital e-mail" and will often say "we determined" with a degree of frequency that is just retarded, where most people would say "we thought/figured/decided".
Also people who put on random accents for some reason. It's usually really shitty cockney English accents as I guess that is easiest for people to imitate for some reason. It isn't quirky or cute, you sound annoying. And slipping random Spanish words into your conversation because you took Spanish I in high school isn't endearing or impressive either.
That makes me think also of people who try way too hard to be perceived as 'weird' or whimsical and ultra-creative. Especially if they are out of high school. If you can make/do cool things, that's cool, show me that. I don't want to hear about how you are just so free spirited because you 'get' Dr Seuss and walk around barefoot in the grass sometimes or you just have such a unique way of interpreting the world or some bullshit like that.