One subreddit that I feel keeps discussion legitimate is /r/askscience
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.