People knee-jerk upvote just as much as they knee-jerk downvote and eliminating one half of the knee-jerks has nasty side effects.
Some people say it gets unlocked after 500 karma, others say they can do it before then which makes me think a mod can activate it for an account.
It was ironic and surprising to see all these high karma accounts discuss a theoretical forum where users can’t downvote without discussing HN’s existing rules.
Frequently downvoted reasons could be added to a blacklist that requires additional elucidation before being able to submit the downvote.
This has been an interesting thought experiment. I'd expect the results might besimilar to Robot9000:
Don't remember how well it worked.