I’m not sure I agree that disabling downvoting really solves the problem. It might help, but not a huge amount. Because you still end up with people upvoting stuff they like and not upvoting stuff they don’t. So instead of being +1/-1 it becomes +1/+0. The stuff that they would have downvoted still ends up sinking towards the bottom, just perhaps not quite as quickly as otherwise.
I do think your thoughts about quote Xits are really interesting though. It’s a two-edged sword. On the one hand, by amplifying what you’re disagreeing with you do also provide an opportunity for more people (rather than less, as on Reddit) to be exposed to it, potentially changing their mind. On the other hand, it’s a tool ripe for abuse and creating more harassment, especially since the people you’re amplifying it to are usually primed to agree with you.