Pretty much just the voting system.
It encourages meta- and shitposting instead of accumulating proper discussion.
It creates a mob effect when people would much rather downvote a comment with negative karma and upvote a comment with positive karma than the otherwise - this is social psychology 101.
Also as
>>7311760 said almost the entire websites are americans. This is a huge issue because there are lots of cultural differences between these retards and the rest of the world, which produces an overall negative experience for both sides.
Picrelated is my filter for the /r/all and it's somewhat incomplete even, the american spam is real there. There's only less than 5 entries that are actually my preference, and the rest is the american "news noise".
Plus as a consequence of having the voting system it turns large communities into a complete garbage unless they're extremely strictly moderated, and, for the most part, don't actuall allow any digital content without the "body text".
Not saying that this place is any better, but at least it doesn't encourage as many psychological fallacies as reddit does.