I think AIBU (and sometimes Chat) is full of posters who think they are brighter than they actually are.
They know how to spell so they snark on those who make typos or can’t spell, whilst missing the point that communication is about communicating.
They spot a hole in a story so pick pick pick at it thinking they’re oh so clever to have spotted it, whilst missing the point that a lot of people obscure details on mumsnet.
They use phrases that they probably think are pithy and witty but actually are just rude and unimaginative, like “are you on glue”, or “you sound like hard work”.
They can’t imagine anyone having a different life to theirs. Particularly abusive relations. I hate the “oh you can’t complain about your mum even though she abused you as a child because my mum is dead!”
Just a lack of emotional intelligence and logic or common sense.
And then of course there’s the genuine trolls who clearly hate mumsnet being mainly female, but that’s a different issue.
I have to say that I read other forums more than mumsnet and the outstanding thing I take from mumsnet is that it gives a poor representation of women.
Conversely to all of the above though there are some posters who post sensible, helpful, intelligent posts so not everyone is an idiot 