Youtube comments currently look like this:
When you upvote a comment, the "thumbs up" symbol becomes blue for you and one point gets added to the score immediately, making it look like this:
When you downvote a comment, the "thumbs down" symbol also becomes blue for you. However, nothing is subtracted from the comment score. Nor does YouTube display the downvotes separately. Instead, it looks like this:
What does YouTube actually do with the downvotes?
I'm assuming it's something considering they have a downvote button at all. (As opposed to the "up only" approach of SE comments, for example, which would be the logical choice if downvotes didn't do anything anyway.)
Does it have anything to do with which comments get displayed and which don't? Is it some sort of under-the-hood stuff that most users won't understand?
Best Answer
Like so much that Google does, only Google can answer. I can find no official pronouncement of what it is for, but there's a lot of people complaining about it.
One conjecture suggests that Google uses it as a signal so it knows what comments to automatically filter. Quoting from the Official YouTube blog:
This makes sense to me. For one thing, nobody likes to see their stuff downvoted. (Witness all of the tempests in a teacup that pop up constantly all over Stack Exchange.) I also think the "thumbs down" is a slightly different signal than "thumbs up". If one person votes up and one person votes down on a comment, meh. But if 50 people vote up and 35 people vote down, that would indicate to me that there may be something significantly wrong with the comment. I think people are inclined to vote up something that they agree with or they find mildly amusing, but probably won't go for the vote down unless it really "bothers" them. (I think this is especially true of a vote down doesn't affect the "score".)
Of course, without an official pronouncement from Google, all you're going to get is speculation.