Hacker News has a moderation policy and mod team that gives the discussion some very desirable characteristics:
- a sort of professional level of conversation is maintained. Roughly equivalent to a casual conversation with a co-worker, over lunch, is sort of the overall tone.
- general rejection of memes and shitposting as being against the purpose of the site -- the point is real discussion an exchange of information, not low effort karma generation
- Paul Graham's essay about different levels of disagreement: generally people attempt to discuss the core ideas of the posts and not just hammer away to trivilaties
Most of the programming related subreddits don't have anything like the overall tone, cohesiveness, or moderator activity/discipline of Hacker News. There are some subreddits that I would say are as good as Hacker News for discussions about their chosen topic, for example, http://reddit.com/r/spacex . But the overall programming subreddits are not equal to this.