I agree that downvoting new people is a bad idea - and every comment in the Welcome Thread should get a load of karma.

However, I think people should aggressively downvote - at the very least a couple of comments per page.

If we don't downvote, comments on average get positive karma - which makes people post them more and more. A few 0 karma comments is a small price to pay if there's a high chance of positive karma.

However, we don't want these posts. They clutter LW, increasing noise. The reason we read forums rather than random letter sequences is because forums filter for strings that have useful semantic content; downvoting inane or uninsightful comments increases this filtering effect. I'd much rather spent a short period of time reading only high quality comments than spend longer reading worse comments.

Worse, it can often be hard to distinguish between a good comment on a topic you don't understand and a bad one. Yet I get much more value spending time reading the good one, which might educate me, than the bad one, which might confuse me - especially if I have trouble distinguishing experts.

Downvotes provide the sting of (variable) negative reinforcement. In the long run, well kept gardens die by pacificism.