My experience of MN is that there are a few people who are plain nasty, but very few. There are (as has been stated above) a lot of "sheep" who repeat what's been said in the posts above theirs. (although, to be fair, often the first 1/2 doz or so replies are all cross-posting and all typing at the same time, so although it reads that way, they're not copying each other [you can see that by the times of posting at the top of their posts]. It's all the ones who come after who do)
It's hard to ignore the nasty ones. I've been described as a bitch, and had biscuits thrown at me, and also described as "you seem like a lovely person". It's REALLY NICE that the internet thinks you're lovely. It's really horrid when the internet hates you.
One of the 'thing's I've found on MN is that you can get 2 more-or-less identical threads, with two completely different sets of answers.
Actually, the 'thing' that bothers me most is that an OP will state something, posters then pile in with their stories and then the whole thread will respond to the OP with the weight of the stories (which are generally extreme). So, to make that more specific, there was a thread a few weeks ago which was about an OP with a nursery-aged child, and her kid might have been a bit boisterous, or might have been a bit mean. Another kid had complained about her kid, but the teachers weren't worried, and the OP was wondering how she should play it. People then responded with their stories of how they'd been bullied (with some really extreme and terrible things that had happened to them) and they changed the tone of the thread, and all the responses became based about THEIR experiences, instead of the OPs. It morphs the thread. So if I post "I've noticed my baby wanks a lot - sticks his/her hand down his/her nappy" and then a poster comes on and tells his or her tale of sexual abuse, then all the responses that come after that are all around how to protect children from sexual abuse. Not saying this to minimise the experience of other posters, but just pointing out how sometimes the sharing of those experiences inadvertently derails a thread. I've had it happen to me, and it's very frustrating when you're desperately seeking advice and instead of helpful hints about how to deal with your DH, suddenly it's DV and you need to LTB! And when you try and say "no that's not it, it's actually just this" THEN the internet turns on you and the Vipers are out telling you you're being U and not listening. Actually, you really WANT to listen, but in order to listen, they need to hear!