Nice! This looks like a better way of preventing long discussions in buried downvoted threads than the previous karma penalty to simple replies.
Hopefully now in reply to crappy posts, people will either create a new thread (or use an open thread) if they have something interesting to reply, or not reply at all if they don't; and people will be less willing to post if they expect to be downvoted - in all cases, it's a win!
Why do we want to encourage spreading discussions around? If it's a good conversation then it makes more sense in context and people should just keep it together, and if it's crappy you're just crapping up more of the forum. We don't encourage good replies to good posts to also post elsewhere.
I just found out that a new website feature was implemented 2 days ago. If a comment is voted to -4 or below, it and all replies and downstream comments from it will be hidden from Recent Comments, and further replies in that subthread will incur 5 karma points penalty. The hiding, but not karma penalty, applies retroactively to comments in that subthread posted before the -4 vote.
This seems to be worth a discussion post since most people are probably still voting things to below -3 without knowing the new consequences of doing so.