> Twitter without downvotes is, after all, famously not a single train of thought under a Tweet!
In my experience, it really isn't! Whenever I see a bad take on Twitter, I generally see some sort of rebuttal within the first dozen or so replies. If you expand that rebuttal, you start to see some argumentation, which to me is the interesting part of Twitter.
I think part of what makes HN's downvote system viable is that it's a well-defined community, you need a lot of karma relatively speaking to unlock the downvote privilege, and in practice comments are only downvoted if they are off-topic or poorly articulated.
In that same vein Stack Exchange's downvote system works well too, because karma (rep) is tracked individually per-community and you likewise need a certain amount of it to unlock the downvote privilege. This makes it so that on SE and HN alike, the people with some seniority in the community are the ones to help dictate what should be surfaced by the site.
On Twitter though, it's liable to turn into the same mess Reddit has. Because there is no hard line between different communities, you can't have a meaningful per-community karma limit for the downvote privilege. This eventually devolves into a free-for-all where users brigade other subreddits (and on Twitter, other users' feeds) to mass downvote opinions they disagree with, no matter how articulate or well-sourced they are.