Imagine how better Twitter would be have they had a "downvote" button like Hacker News!
I mean how many times getting downvotes taught you how to behave on HN?
Long time ago I once posted a joke; it was good and funny joke even somewhat related to subject. I got downvoted into oblivion. I scratched my head for a while but then realized I do the same thing - even if I chuckle at someones good joke, I cast my vote of thumb down to remind them in order to keep quality engagement on HN, don't post jokes! I eventually figured out it was a venue thing. You may be the funniest guy in the word, but if you go to church people expect you to remain in silence and/or pray, not make jokes.
Twitter used to be really great and then few months ago I just gave up on it. I follow few people that spew hate rhetoric. I watched them posting their stuff about Clinton's emails over and over again. Sometimes getting stupid answers, sometimes being ignored and sometimes actually seeing people putting time and effort to answer and explain. These individuals were not interested in your response! They were there just to post their rhetoric like a contest who scream louder in empty echo chamber! I know that HN downvote button would help cleanse Twitter because with few thousands posts per second, obviously nobody will decide manually how valuable the tweet is.
Being downvoted on Twitter would mean first your posting frequency goes down. Then it could mean your post is sliced to 210 char (half of original 420). Finally you could be jailed automatically for 3/7/14/31 days. I mean there is so many ideas to get folks post quality content or GTFO that its surprise management allowed Twitter to slide this low. I am sure I am not the only one who gave up on Twitter, or perhaps to Twitter bottom line (ad $), it doesn't even matter.