I do think that you're right that there aren't really solid norms in place for how to interact on AO3 in the same way that there were on LJ. This is something I'll need to pick apart more.
On this front, I think a lot of this is due to ao3 ending up as a single fandom hub, because you have a metric fuckton of different communities in one space, and the site, on a code level, was biased towards certain communities from the start (I don't think intentionally - the founders built something that fit *their* fandom community). You see it the most in stuff around tagging - all the issues with synned tags in the Minecraft Youtuber community, or debates about whether you should tag a background ship (so people can avoid NOTPs) or not (so people looking for that ship can actually find content that features it. But comment culture plays into that too. I'm friends with a moderately popular writer who's been writing in various fandoms for twenty-plus years now, and some of the things people say in comments on ao3 are... wild. Like 'why would you think that's an okay thing to say to a stranger'-wild.
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On this front, I think a lot of this is due to ao3 ending up as a single fandom hub, because you have a metric fuckton of different communities in one space, and the site, on a code level, was biased towards certain communities from the start (I don't think intentionally - the founders built something that fit *their* fandom community). You see it the most in stuff around tagging - all the issues with synned tags in the Minecraft Youtuber community, or debates about whether you should tag a background ship (so people can avoid NOTPs) or not (so people looking for that ship can actually find content that features it. But comment culture plays into that too. I'm friends with a moderately popular writer who's been writing in various fandoms for twenty-plus years now, and some of the things people say in comments on ao3 are... wild. Like 'why would you think that's an okay thing to say to a stranger'-wild.