There’s always been an element of rudeness or nastiness here - and I’ve been around for almost 20 years.
It manifests itself in different ways, though. Back in the day it was very cliquey, with constant calls to ‘MSN, now!’, making it clear that you were either ‘in’ or ‘out’.
As the site has grown, the demographic has also changed and it’s now much more reflective of the whole population, I believe. So you get a much more diverse range of views. And that can provoke strong responses.
One thing I do see more of, though, is people calling ‘bully’ when they’re being disagreed with. Along with the expectation that we should all #BeKind, which is never going to happen! The joy of being online is the freedom to express ourselves genuinely, without having to couch responses such that we don’t offend people. Online interaction is very different from face-to-face interaction, which makes it more direct. Which can be seen as blunt or rude.
As PP have said, if all you read on MN is AIBU, you’re really not seeing what posters are actually like. There’s a huge amount of support here on the various boards. AIBU exists to provoke argument and dissension, therefore it can easily be taken as being nasty or rude.