@KMW79 I think the majority of people are really nice, but there is a hardcore of (a few) people on AIBU... I reckon there's about half a dozen... who are just mean and spiteful for the sake of it. The same 5 or 6 posters pop up on lots of threads, bashing the OP when they're down, and using very typical mumsnet-type phrases.. 'are you on glue' you sound unhinged' and 'why did you have children with this man,' and 'you don't sound very bright.' And other snarky, rude put-downs designed to make people feel even more shit than they already do.
Apart from the usual 5 or 6 suspects, you often get 'new' posters popping up to attack posters too. And I have seen someone post on a thread purely to attack a poster, and for no other reason. No other contribution to the thread, just that attack.
You just know which posters are going to post nasty/goady comments too. Whenever I see the last poster on a thread (when looking at the front page of the threads,) and I can see it's a certain one of about half a dozen posters, I KNOW it's going to be an unpleasant/goady/unhelpful post that will almost certainly be attacking the OP (and/or another posters...)
It is only in AIBU though, and occasionally in Relationships, and Chat. And for the most part, mumsnet is fab. So many lovely, funny, intelligent, wise women on here, who have made me feel a LOT better about myself, and the shit going on in my life. 
I also don't 'get' people saying 'leave then!' It's like when people say Coronation Street is a bit shit these days, it used to be so good in the 1970s/1980s/1990s, 2000's, and someone says 'well don't watch it then!' 
It's not always as cut and dried and that. When someone loves something and takes part in it/watches it for many years and then it goes shit (in their opinion,) it's not because they want to bail, they are complaining because they want it to IMPROVE... Do people seriously not get that?
YANBU OP.