There's quite a few but I'll give my top 3:

r/TIFU and r/AITA - The former became a repository for preteen fanfiction and the latter became a place for confirmation bias/rhetorical questions looking for validation.

Then there's r/UnpopularOpinion which ended up being an oxymoron unto itself. I honestly don't understand how anyone thought that concept would work given that the literal point of a social media discussion platform, that utilizes an upvote/downvote system to determine visibility, is to push popular (highly upvoted) posts to the top/front. Very few people actually upvoted something that was unpopular and instead just upvoted the low hanging fruit popular opinion posts that were 'controversial' but still blatantly have a clear majority who support that side that OP took.