Gatekeeping communities. A lot of the niche communities basically would push you away if you don't have the latest or greatest, or if you had a slightly differing opinion than the rest of that community
Alternatively, one of my favorite things about Reddit was the circlejerk communities which were half tongue-in-cheek about gatekeeping certain things.
/r/guitar was known for being weird about their guitars, to the point you'd think they wanted to fuck them. /r/guitarCirclejerk was great because they'd ham it up and take it even further. /r/VinylCirclejerk was the same, they'd just post pictures of their feet with a record way off in the distance and say "how's my collection?"
It's semi-gatekeeping, but it provides, in my opinion, a funny counter to some of the more serious subs, or just makes fun of people who post dumb things.