I think you touch upon a great area: There are many ways to express agreement and disagreement, but downvotes being used as a form of suppression are neither.
In the before times slashdot had a metamoderation system where users were randomly assigned a few votes they could apply, the rarity and distribution made for a reasonable approximation of a fair moderation. However, lemmy differs from slashdot in that there are many different unaligned communities on lemmy where slashdot (and hacker news, and lobsters) are basically a single community with very clear unified interests.
The keys for high quality discussion (not agreement) in a community would be (best guess):
- Participation is not chilled, even of unpopular things as long as they follow the community rules
- No ad-hominem attacks in the discussion
- People of mutual levels of energy and engagement can find each other. (the people who like to cite papers vs the people who like to cite youtube videos)
- Little to no grandstanding, not talking past someone just to repeat your point for third party viewers
I suppose what I'm describing is the framework for a debate society or even toastmasters.