Trouble is, without strict moderation the stuff that floats to the top will be the memes / jokes / lowest common denominator echo chamber opinions, not to mention the people trying to game the system. Look at most of the default subreddits, for example.
Not sure whether you comment above is directed at Youtube only, but Amazon does have "Was this review helpful to you?". I believe voting on Yes makes it more prominent. As usual, this kind of system is subject to fan-boy treatment. I once left a 1 star review of a game which I genuinely disliked and gave my reasons (involved tinkering around graphics settings in XML files); there were a lot of clicks on the "No" button to the above question, presumably because the fan-boys could not tolerate someone criticizing their favourite game.