My parents were Grammar Nazis, so my spelling and grammar are generally good. Except.. well.. I've never met a comma I didn't like. I use them as I would speak, generally. If I'd pause when saying it aloud, I use a comma. I realize this is incorrect, but it's a reflection of my speech.
The "it doesn't matter because it's the internet and no one's grading me" annoys me. Mostly because, as I was told in school, "The way you practice is the way you play." If you can't be bothered to type correctly when it doesn't matter, you often won't think twice about typing things incorrectly when it does matter. I give allowances for typos and even for poor spelling.. mostly because I've come to believe that spelling is a more or less inherited trait in some ways - meaning, I've known people (like my sister) who've never been able to spell worth a damn from the time they were young and people (like me) who have never had a problem spelling much of anything.
My biggest problems are then/than, your/you're, to/too and there/their/they're. Yes, I realize one of those isn't a "complete set," but these are the actual mistakes I see most often. There are easy to remember rules to follow to make sure you're using the right one(s).
As for "teh," I only use it for comic effect and self-deprecation.