I remember exactly why I joined, incidentally. With increasing regularity, /r/programming was making me very mad. I would post a detailed correct answer to someone's question, only for some troll to immediately reply with some insult and "proof" that I was totally wrong, even though the proof was totally wrong. It happens once, fine. It happens every single time I comment, I'm out. I have not been back in 8 years.
I will admit this happens from time to time on HN, which is unfortunate because I like to comment and really only know what I'm talking about when it's programming-related. But it hasn't made me mad enough to leave, only to nostalgically think back to the good-old days.
(I did start using Reddit again, too, but I only read unpopular subreddits. I can handle /r/AskReddit for some time killing, but things like /r/flying and /r/amateurradio are very pleasant communities with enough activity to be interesting a couple times a week. /r/anime is infuriatingly stupid, /r/awwnime is much more tolerable.)