I've read AMAZING threads on here. Gloriously funny (like... literally laughing out loud). Incredibly helpful.
Also read pile-ons, which are generally horrible (I've always wondered who would actually go and watch public executions, like they did in "history". The answer is many posters on here).
Added on from that, groupthink. It's horrible. OP posts something, an early poster states strongly "it's X" and poster after poster just adds a variant on that.
On the same theme, I've posted about things and MN has misunderstood and will NOT be persuaded otherwise. That's very unhelpful.
On a lighter note, finding teens getting drunk is NOT considered funny on here (got my arse handed to me on a plate for that one under a different name).
S&B can also be vicious (I got roundly told off by them, also under a different name). Thought only AIBU did that. I take this as a badge of MN honour.
That some people clean to obsessional levels and think it's normal.
That some people ask for help for the most stupid things (I once actually read a post for someone asking how to book into a busy exercise class that was always full. Like the internet would KNOW that??? Obvs the poster couldn't say "it's the 5:00 aerobic class in Uxbridge" because that would be too outing. Um... so how can we help you?)
Having said all that, I've seen MN be so helpful and caring it's actually taken my breath away.
Oh, and (maybe, for sometimes perfectly innocent reasons) posters often embellish and lie. Maybe not for perfectly innocent reasons. There was a thread about "show a picture of your bedroom" and posters were putting up pictures of magazine bedrooms and pretending it was theirs. Like... who, exactly, are you trying to impress???? Crazy!