Maybe not subsections, but allow users to create and up-vote tags on stories, or maybe have a simple dropdown on story submission with a dozen or so tags the submitter could apply. That way you could still narrow your focus when you don't want to miss certain content (ex. #Python, #AI, #Business, etc. at news.ycombinator.com/tagged/Python, etc.), but you wouldn't disrupt the current community feel of HN too much.