A few of my least favourite things:
PP who don't RTFT - they often post something completely irrelevant because the thread has moved on and it's obvious they haven't read it.
OPs who flounce when a thread doesn't go their way and ask MNHQ to close the thread. They put something absurd like too much RL info given when in fact they gave none.
Posters who support the OPs no matter how wrong or ridiculous they may be.
Posters who try to excuse any behaviour, no matter how awful, by suggesting MH issues, autism, or any other excuse they can think of.
OPs who drip feed important info 3/4 of the way through the thread
OPs with lazy DH/Ps who have never nor will ever do housework or childcare and the OPs want to know how to get him to change. He won't. After listing their many faults they invariably describe the DHs as 'good fathers'. No they aren't.
OPs who post something like 'my eyeball is hanging out of its socket on a thread - should I go to A& E or wait until I can get a doctor's appointment the week after next?' Use your bloody common sense!
Dredging up zombie threads.
Endlessly repeated threads about P&C parking, how often you wash your hair/sheets/towels/PJs/loo brush etc.