Tags: much of the content of Reddit is people reposting the same thing at various times to various subreddits, and many of the comments are people noticing that it fits into "r/whatever" instead. The default behavior should be that posts of the same link to various tags (r/awww, r/catsstandingup) should not recreate a new post but just backlink to the same post. Repeat posts to the same tag should be a no-operation. This removes the community aspect of tags, and some of the worst emergent behaviors of reddit along with it. Treating a tag as both a topic and a community leads to a neverending cycle of people creating niche topic tags for the community aspect, but then finding themselves "invaded" as people will always want to find and comment on content about topics that they disagree with.

But of course the community aspect of Reddit leads to good things. So you make a separate concept of communities. These communities could follow the posts of one or more tags, applying filters to those of their own choosing, and make their discussion visible within community borders only or visible to all, based on their own choices (in addition to internal community posts.) They can control admission to the communities on their own rules (fully public, invite codes only, etc.) The ability to automatically follow the posts of certain tags (and either view or not view public comments) will keep small communities from getting stale for lack of posts.