I moderate a medium sized Reddit community (r/mtb, ~32k), and the key for me has been enforcing a few basic rules: no memes, no rage comics, and all posts must follow reddiquette. (Okay, that 3rd rules is really a blanket for 10 rules, I admit.)

Sometimes we get trolls, but because of the rule structure they usually get bored pretty quickly. I have a handful of regular posters, too, who do a good job of posting every day and reporting spammers. It really has a true community feel, as was the intention of the site.