As Reddit has grown from a small, insider community to a highly-trafficked intersection of conversation and news, debate has raged over what should get shown on the site's front page. Like Google search results, the first posts will have a much better chance of getting seen than those with lower popularity. Eventually the site got big enough for the larger subreddits to outperform the new ones cropping up, so Reddit admins assigned a set of default subreddits to fill the front page, effectively choosing what type of content logged out users see. To avoid this editorializing, they've started fresh with a new concept: r/Popular.
This new subreddit will populate the front page with a diverse set of content, like the old r/all used to do. Logged-in users won't see any change, as their landing page will still be filled with posts from their subscribed subreddits. Visiting r/Popular shows a decent amount of content from the larger, recognizable communities, but with some new blood trickling in, like posts from the inimitable r/NatureIsFuckingLit.
Reddit has been wanting a more elegant solution than forcing a set selection of content in front of logged out users. The r/All subreddit was an imperfect solution, as evidenced when the site gave users the ability to filter out content from communities they didn't like. While r/Popular is more hands-off for administrators, there's still a palatability check: Content won't show up from NSFW and 18+ communities, subredits that have opted out of r/all and those that users constantly filter out of their r/all page.