Ask HN: What are the forums interesting and meaningful discussion is going on?

I find some of the subreddits interesting and similarly, few people write good answers on Quora. Those are of the kind which makes you think and you feel, you that wasn't feeding junk-food to your brain (even if they weren't on the topics you are not consciously interested in). But most of the threads on these sites are filled with junk (though I will be first to confess, I spend a significant part of my day browsing them). What are the sites where a majority of items and conversations are meaningful and people are not replying just the sake of it?