You have some rude and unpleasant people on here who just jump on ANYone and start being nasty, when all they need is help and support.
Then you have the muppets who ask 'AIBU?' who start getting sniffy, when some posters say yes, by saying the ones disagreeing are nasty bitches, whilst giving a list of reasons why she's right, and they're all wrong.
This meme I am putting a pic of ^ is SOOOOOOOOO mumsnet though. 
As a few posters have said though, I don't think it's any worse than it used to be. I think people forget that some people on message forums have been nasty, judgy, spiteful, and condescending for YEARS. (Including here on mumsnet.)
I used to be on ivillage, and loose women chat forum, and also imdb, moneysavingexpert, and a chat forum of my favourite film star .. I am talking around 15 to 20 years ago for ivillage, 10 years or so since the Loose Women board, and 7 to 10 years since I was on imbd, moneysavingexpert, and my favourite fim star forum,...
And there were nasty, rude, condescending judgy, spiteful posters on there even back then...
It's always been the same on message forums (and social media.) As has been said, I think people feel safe behind their anonymity, and say things they would never say to someone's face. You also get judgy, spiteful, bitchy posters on twitter, and reddit, and facebook, and instagram, and different fan boards for various celebrities.
It's always been this way, and it always will be.