Books generally collect lots of upvotes, but few comments, since you generally have to read the book first before commenting. Controversial news articles, whos

Books generally collect lots of upvotes, but few comments, since you generally have to read the book first before commenting. Controversial news articles, whose entire content can be summed up in the headline, collect both upvotes and comments. The ranking algorithm punishes submissions with many comments, to push those submissions off the front page.