Reddit is moderated by mods, who are independent of the site.
Any random space alien cat can become a Reddit mod, and does. You've got to be hired by Reddit directly to become an admin.
The philosophy of Reddit is to allow subreddits to operate with very broad autonomy, save a few specific behavioural exceptions. There have been instances (spez's rewrites, for which he's apologised, of a small set of abusive comments), otherwise, but the overall philosophy is hands-off.
HN's is very much hands on, though it's mostly gentle guidance: admonitions, thread detaching, and if necessary, banning accounts, though pretty much always after a warning. If you go through dang and ... the other admin's accounts (sorry, other guy, but that's entirely nonmemorable), you'll see a great deal of activity. HN reflects the founders' and adminstrators' views far more than Reddit.
Mostly, that works out well, though it clearly doesn't scale to the extent Reddit does. This is not all bad.
Also - sometimes it's refreshing to see an interesting post and not have to see replies about how someone is an expert at estimating the weight of rabbits based on the size of their hole. Or read about "orders of magnitude" of this or "straw-man" that.