One subreddit that I feel keeps discussion legitimate is /r/askscience. The moderators rule with an iron fist and mercilessly remove inane content and logical fallacy. I like that subreddit a lot because of it. But I agree, the overall quality of Reddit is now akin to the way Digg was right before the site redesign. Terrible.
That's pretty much one of the few I was thinking of as well. With those kind of numbers, heavy moderation is the only way to go. In other big subreddits whenever the mods propose some new rules and changes the community throws a collective hissy fit of "OMG THIS IS "OUR" SUBREDDIT DON'T TOUCH IT IT'S FINE. LET THE UPVOTES AND DOWNVOTES DO THE MODERATING!". But up and downvotes really don't work anymore, and are biased towards image/meme content because of how the system works.One subreddit that I feel keeps discussion legitimate is /r/askscience
And of course the availability of upvote-bundles which you can purchase online for marketing purposes. I felt an heavy uprising in PR-related content in the past year.