Topic hierarchy involving either vote based sorting, temporal sorting, or personal relevance sorting. e.g. forum -> subforum -> thread; HN's comment system. Yo

Topic hierarchy involving either vote based sorting, temporal sorting, or personal relevance sorting. e.g. forum -> subforum -> thread; HN's comment system. You can extend it to allow multiple topic parents via a tagging system, but this usually seems to complicate things. facebook has wall -> post -> comments, but could add richer hierarchy above post