The threshold ought to be a user preference. I might consider starting to have a policy of upvoting all comments currently at -4 or lower (except stuff I would delete if I were an admin, i.e. obvious spam, disclosures of confidential information, and the like) until it is made into one.
The threshold ought to be a user preference.
If so, then definitely of the 'opt-out' kind with the current behavior as the default. Making the (at least reasonably likely to be) trashy conversations invisible to new users is one of the most important goals of the feature.
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.