Per-thread (or per-tree) makes a lot of sense. I would guess that relevance and quality are best judged locally, rather than globally. There are definitely glob

Per-thread (or per-tree) makes a lot of sense. I would guess that relevance and quality are best judged locally, rather than globally. There are definitely global principles on which to assess the quality of a comment. But even those principles depend on local context. There are certain categories of comment, in particular, that lend themselves to mischaracterization when viewed out of context. Humor, for instance. Sarcasm. Subtle parody. Tongue in cheek. Ideally you want more from a comment than just a snarky one liner. But sometimes a well-placed quip adds perspective and value to a conversation. (Most of the time, it probably doesn't. But you don't want to implement a system that filters all of it.)