First, for transparency's sake, I actually struggled with this question as I've held a lot of misconceptions over the years. I've been in fandom since 2000, after all. I'm also not entirely sure what I've covered before.
But I think one, which was touched on in
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I even corrected people on forums I belonged to, and when I say I didn't see anything wrong with it, I really mean it. I thought I was saving them, the way terse forum mod sonce "saved" me. Except what were mods saving me from? What was I saving those people from? Nothing. Absolutely nothing.
It was also common to make fun of individual fandomers, whether their fics or just who they were in fandom. Often the comments were way too personal, especially when it came to fic writers. Even when it came to people who deserved personal criticism, sometimes comments still went way too far.
It's interesting. This isn't really fandom, but stick with me here. I was thinking about the site STFU Parents, which made fun of the way parents (usually moms) talked about their kids on social media. I remember feeling like the blog was very harsh, and I recently gave it a re-read. While there were certainly awful posts that deserved to get sporked, a lot of them were just... people daring to talk about their kids instead of whatever was "cool" to like on the internet at the time. Sometimes they'd USE ALL CAPS or word something a little awkwardly. Okay? It happens. And no, I didn't care about people's kids either, I was online to geek out about stuff, but I also realized Facebook wasn't where geekery was happening. (I'm not saying that people necessarily thought that, either, I'm just. Saying.)
Even the reaction to Rebecca Black's "Friday" is arguably part of this idea that the internet had to be protected from... something. You're Wrong About (I think) had a whole episode about that, where they pointed out that those kinds of videos were outliers on YouTube back then. I get why Rebecca Black stood out like a sore thumb, but let's maybe not make fun of thirteen year old girls? That bothered me too. (And can I just say, Party in the USA, released around that same time, wasn't a whole lot better? Miley Cyrus just sings like, well, Miley Cyrus, whereas Rebecca Black just sang like a teenage girl. Because she was a teenage girl. Maybe Glee made us forget how most teenagers sing.)
So yeah. The internet is a lot worse in many ways, unfortunately. But I'm glad I can read YouTube comments now without losing my faith in humanity, I'm glad people aren't flamed on Ao3 as frequently as they were on fanfiction.net. (At least in my experience.)