The easiest one would simply be removing the down vote mechanism. If with an existing site then can compare with the years' prior stats - assuming they've been recorded; I don't know if HN does. There are a number of things you could monitor - would be interesting to see if any sets of people with the same behaviour change their behaviour more than just down voting, like if they stay on the site more or less, if they comment more or less; perhaps doing word use analysis on these different sub sets before and after the down vote change, what language are people who are prone to down voting using vs. those who only comment or only up vote but don't down vote, etc.

Not op, but one idea: Ask for an articulate reason for downvoting. Increases the barrier of downvotes, and if people don't feel that it's articulate they could then downvote the downvote (with their reasons why).

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: https://blog.xkcd.com/2008/01/14/robot9000-and-xkcd-signal-a...