I almost forgot my all-time biggest complaint:
People who quote the ENTIRE original post in their reply.
Quoting the part that they are referring to is fine and helps clarify their point, but reposting the entire original post defeats the purpose. Don't you think people read it the first time through, or can go back and re-read it?
(HINT: When you use the quote function, you can delete that which is irrelevant.)
Of course, this one is doubly annoying to me since, with my vision impairment, I have to zoom in the text 200% just to read. Having to scroll through endless renditions of the original post makes me want to run screaming into the night...Not that it didn't bug me when my vision was normal, LOL...