I have OCD, so my answer to this depends on whether I've had my meds or not.
One thing that gets me no matter what, though, is lack of punctuation. Not everyone has good grammar or spelling, and that's ok as long you're still understandable, but if you never even put a period at the end of your sentences, I honestly can't read what you wrote. I just can't. It isn't so much that it annoys me as it is that I just have to skip the post and hope it wasn't important.