- If all people are polite and do not like to vote down then, when someone says something crazy it will be left unmolested by the large multitude who see and recognize the craziness.
- If a person writes something reasonable and complicated, most people will give up reading it as “obvious” and “too long”. It will receive no likes and no dislikes.
- If a crazy person writes something crazy, short, and pithy, other crazies will up-vote and bring that to the top of the pile, to influence people who are on the edge of craziness.
- If left and right crazies both do this, then all discussions at the top of the pile will be crazy nonsense, with tons of likes. Reasonable and insightful responses will be banished down below.
Ergo: If reality matches this prediction, then “vote up” is a bad feature and “vote down” is strongly underutilized and it is the fault of politeness that allows bad actors to thrive and spread.
Part of this is also compounded by the fact that many platforms, such as Facebook, don’t **have **any thumbs down option at all either. So no matter how crazy a post is, you can find some people who will give it thumbs up.
Vote up? Down? Where? Are you referring to something in particular? Can you explain please?
Somebody figure this out before I accidentally get a gladiator killed.
Where are we voting up or down?
But what is he talking about? Is this some joke I don’t know about? 
In my experience with Reddit, IMGUR, and sites using Disqus, the OP is right about one thing.
Up and down votes aren’t some arbiter of objective truth or quality, because it’s impossible to democratize that.
But the rest…
While unremarkable posts and replies will be left alone without votes (in some communities, but not others), ‘unremarkable’ is not the same as ‘not crazy’.
What gets primarily upvoted - things the community feels is important to amplify, jokes they think are amusing, posts that are well-thought-out in the community’s opinion (which may or may not agree with your opinion), things the community feels are interesting. IE: Stuff the community at large feels is remarkably good.
What gets primarily downvoted - things that are offensive to the community, jokes that fall flat, posts that are wrong in the community’s opinion (which may be objectively supported or not), things that are off-topic to the community (in some communities). IE: Stuff the community at large feels is remarkably bad. If only the crazies get upvoted, then it’s a crazy community. If it’s not, then the upvoted posts will be sane - possibly wrong, but within the normal parameters of wrongness.
So, what what gets voted in which direction tells you is not the quality of the post - even if you reverse the meanings of the votes - but the general personality of the particular community you’re looking at. If only the crazies are getting upvoted, then it’s a crazy community.
Youtube, Disqus (the comment engine used by most websites), Facebook (historically), Reddit, and other sites on the Internet use up votes and down votes as a way of sorting the information rather than, like the SDMB, keeping things chronological. Up votes go to the top and down votes go to the bottom. Up-votes are probably about 10 times more common than down-votes.
I recommend, if you are ever on any of these sites, to work to reverse that.
If you mean this quote, it’s referring to the controversy about which gesture were used by the audience at Roman gladiator events where they get to choose if the defeated opponent lives or dies. It might be thumbs up for life and thumbs down for death, but could be a whole bunch of other configurations and the truth is uncertain.