In my experience, it is the upvote/downvote mechanic in combination with critical mass. I was a long time paid subscriber to a popular tech/science news site that had an active comment section and a moderately sized active user base. I really loved it, checking in several times every day as new articles were posted. The discussions were great, even though I commonly disagreed with many users point of view. Posts felt genuine and ideas were well thought out and defended.

I like to think that because of this, the site grew in popularity. As it did, the comment section degraded in post quality and thoughtfulness of responses. The sites news editors more and more catered to their audience, and the quality of the articles likewise declined until it was one giant groupthink echo chamber, all chronologically organized without using a tree system.

I gave up and unsubscribed. I would like to try removing the arrows from forums so that no one can offload their thinking to the group. Everyone will be forced to decide on their own if a post is good or bad without the benefit of the group telling them what to think.