As an aside, I think it's notable how Metafilter has maintained a high-quality link aggregation/discussion forum over many years through the use of community norms and relatively aggressive moderation.

That analogy doesn't account for what happens when a submission gets posted to both a "pinot" subreddit and a "sewage" subreddit, and subscribers in the sewage subreddit see the post in the pinot subreddit from the other discussions tab and "invade" the other subreddit.

Subreddits and their communities are not neatly contained in clearly labelled bottles.