Plagiarism is something I totally support the use of downvotes. Cycle voting is not gonna stop even if we do, and I think downvotes should be used for the following reasons exclusively.
Racism, hate speach, plagiarism,
The cycle voting could be really easily fixed but is there a will for that?

If you vote for someone that you already have voted for and has pending payouts from your vote your next vote value should be -50% stacking up eventually to zero. It will probably be a valid reason to stop the cycle voting or at least limit it to a degree and gives a chance for more engagement and more curation. Which is the most important aspect here.
Everything else probably is fair game if you take into account that everyone is doing it from what I have seen so far. I do tend to vote those that support me more than those that don't, unless their content is exceptional good. Something something game theory.
I also just upvote content that I know a whale will vote for it even if I get to disagree with the said content. Let's say conspiracy theories or fear-mongering in general which is something that people really support for some reason. I will upvote it cause it will return revenue to me so there is that.

I will add downvotes are not a tool to resolve into an argument, if people cannot handle different opinions they shouldn't be here in the first place. Expressing a radical ideology it will get controversial really fast happened to me yesterday. Just because I upvote someone that does not automatically means I agree, In that case I will say why I think he is wrong he is welcome to correct me, but when that someone claiming the journalism tag requests from me to stop commenting on his posts I will have to call bs and it is a valid reason to down vote.

I am not going to take a lot of space, I will probably make a post in the future about it but I will also downvote someone that is exploiting the reward pool constantly.