Recommendation engine pushing users into ideological bubbles, public voting mechanism creating incentive for conformity which then creates purity spirals, lack of moderation.
It would surprise me if the winner in that space didn't have a public voting mechanism. Digg, Reddit's early major competitor had one, and heavy-handed moderation surrounding the HD-DVD decryption key leak was one of the major inflection points that drove users from Digg to Reddit. Stricter moderation during that time period would have been a losing strategy.