Karma scores - on an account level at least. Up/down votes on a post or comment are fine and make sense, pushing bad replies down and the best, most thoughtful stuff to the top.
But a system where accounts can build up a karma/reputation score just leads to karma whoring comments just intended to gain upvotes and adding little to the conversation. Or worse, repost bots just reposting whatever was popular last week to gain karma. Reddit's been plagued with it for years and it just makes the whole place seem spammy and low quality.