That's a great idea. Maybe it needs good auto-categorization to implement well, but another idea is to not bother with the thresholds at all -- every comment can potentially spawn off a new wave of replies, so why not just fork into separate threads immediately? The top level view gives you the flat view of the top level thread, and by diving in through the replies you can keep flat views of each sub-conversation. If every comment is treated the same, then it truly doesn't matter how old the comment you're replying to is because you could be sparking an entirely new discussion.