I'm fairly new on here, and overall I like it (fairly obviously, or I wouldn't have hung around ).
The things I find irritating on here are the same as those that niggle on other forums and Facebook groups. I'm not sure that anything can be done about them, really, and it doesn't put me off posting if I want to:
People who don't read the thread, and 'join in' by repeating what has already been said. It's rude to those who have taken time to say something that is ignored and then re-presented as someone else's idea, and it's infuriating to read the same thing over and over, particularly if it's already been explained several times.
People who talk to one another over the top of a discussion. It's natural to make online friends, but there is a PM facility if people want to have private conversations.
As has been said, OPs who disappear after asking a question. I don't think that people should feel that they have to respond to every post, particularly if they are feeling 'got at'; but it is bad manners to get a lot of advice and ignore all of it.
When it comes to the act of posting, one thing that I find maddening it the lack of an edit facility. I completely understand why it has been removed, as if posts can be edited at any point, people can alter theirs to make others look bad, or otherwise cause mischief, but it could be set up so that their is a 2 minute edit window, or so that posts can be edited until there is a reply, or so that edits can be viewed. Having to explain every typo, or try to work out other posters' interrupts the flow of reading, and gets in the way.