Long OPs full of irrelevant info.
Cancel the cheque posts - please just read OP's updates and in some cases just read the OP thoroughly.
Posters who get snotty or nasty right from the off for no apparent reason - often with vulnerable OPs and if anyone tells them to tone it down a bit they get nasty with them too. You wonder what's going on in their lives to do this or if they are latent bullies.
Posters who take an opposing view to others but won't say why and get snippy when asked. "I don't HAVE to explain myself!" - no you don't but it makes the thread and your posts in particular pretty meaningless - "well it's that way because it is as that's what I think so there" is a pretty weak argument.
My personal bugbear are posters who use extreme and skewed language to try to make their point. People who don't agree are "deranged" or their situations must be awful or they must accept awful behaviour - no you just disagree. If you have to falsely ramp up your language to make a point it is probably a pretty weak point.
Really worrying posts where the poster doesn't respond after the initial 2-3 posts.
OPs who post about negative behaviour towards them, ranging from appalling to just a bit off and are given good advice that they can't take because of xy&z. It's frustrating for posters advising. I take it they just made the thread so that it will be confirmed that OH/DC/DP/DPIL/DB/DS/DFriends are total sh*ts but you are going to do bugger all about it.
I'm sure there are loads more