Imagine you're at a dinner party, and you're getting into a heated argument. As you start yelling, the other people quickly hush their voices and start glaring at you. None of the onlookers have to take further action—it's clear from their facial expressions that you're being a jerk.
In digital conversations, giving feedback requires more conscious effort. Silence is the default. Participants only get feedback from people who join the fray. They receive no signal about how the silent onlookers perceive their dialogue. In fact, they don't receive much signal that onlookers observed the conversation at all.
As a result, the feedback you do receive in digital conversations is more polarized, because the only people who will engage are those who are willing to take that extra step and bear that cost of wading into a messy conversation.
It's a great post, and has a really solid UI idea in the footnotes.
One idea I'd really like to see platforms like Twitter or Reddit try is to provide a mechanism for low-friction, private, negative feedback. For example, you could imagine offering a button where you can downvote or thumbs-down content (i.e. the opposite of a Like), but the count is only visible to the OP and not to anyone else.
The LW team has been thinking about building private responses like this for a while, but in comment form. Buttons that give more constrained private info are very interesting...