The comments from knowledgeable individuals - frequently involved in the post itself. How often did I read of an astronomical paper, only to have one of the authors comment. Or read about some random fact about plumbing or medicine or whatever, and an academic or professional from the field would offer further insight

Not to mention the spectacular recommendations in various areas: whenever I'm in the market to buy literally anything, I'll search for the best of it on Reddit. The amount of high-quality information available on Reddit is not easily replaced. For that reason, I'll probably continue making such enquiries there, even if I do give up on Reddit in every other way