Is it? The subreddits I follow do have some quality discourse, though there is a bit of a "hive mind", where you have to say the same things as the majority if you care about your internet points. So I wouldn't go as far as implementing a point and reward system, but just upvotes or downvotes to highlight or hide threads, as quou suggests, would help. That seems to be the way it's already implemented in the Release Announcements subforum. Because the itch.io forums are indeed filled with immature and useless content, it often makes me wonder why I'm still occasionally checking them...
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Reddit is garbage.
The main problem with arguing that their system works for a few handful of subreddits is that you're implying that those "quality" subreddits are an exception, rather than the rule. In other words, it's not a merit of the hivemind system Reddit uses that keeps those subs from being like the rest of that toxic hellhole, it's a merit of those moderators for running their subs differently than the usual hivemind clout-chasing spam that plagues the defaults (until they stop, and the sub "dies" from getting main-site traffic).