The biggest problem on reddit is having both up- and down-votes. That allows the majority to effectively eliminate dissenting opinions on any topic it cares about by down-voting them to oblivion, and then pat itself on the back for the fact that everyone apparently agrees with it. Since it's possible to do that, some see it as an obligation and go at it with gusto, making it hard to have a conversation that strays outside the current-year party line.

Systems which only have up-votes/likes have their own issues, but at least not that one.