There might be discussions that transition between censored and uncensored.
This could be annoying, if we have to check all the comments upstream from the one we're responding to, to make sure there isn't a comment that might be downvoted to -4 in the future and make the effort a waste. I pointed out a bunch of potential downsides of this proposal to Eliezer but even I didn't think of this one.
Yeah, most of the systems that do this sort of thing seem to hide the low-scoring comments but show high-voted children, avoiding that sort of problem.
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.