In all seriousness, it is the atmosphere more than anything. Friendly, welcoming, and informed membership. Good banter, can take a good shot as well as give one, where you never feel you have to walk on eggshells from the site owner to the greenest n00b. Good debate, good discussion, and good laughs without holding grudges after disagreements.
Beyond this, you want members like Darkbulb who are always up to date on the latest hardware or software, who will give membership the skinny first, and you want collectors like Billy that have a wealth of information on vintage stuff (and shimming, etc.), guys who restore old straits... all that, and pass arounds and PIF's that share their thoughts and are always thoughtful of the next guy.
Moderation should be blind to their friends and enemies when making any decision in this capacity, always choosing what is right, not who is right. And they should have a very light touch, with potential warnings and explanations in private. Mods, I feel, should also be rotated among senior membership for a set amount of time, then passed on to another so there is never any entrenchment. The site owner/admin is best left to stay out of all moderation duties, and only decide when there is a disagreement among mods.
Never, never charge for shaving related artisans or vendor participation or advertising on the site! This only eventually leads to a protected status among paying vendors, and then they have a say that is disproportional to non-paying competition, and even regular membership. Welcome them aboard as shave enthusiasts just like us, with the warning that they cannot spam their product, but are welcome to answer questions where other members are interested. While they have other problems, the DFS forum has the perfect model for vendor participation, and we should emulate that model. More artisans post on DFS than on any other forum, and they do so as regular guys, without spamming, but their accounts are clearly labeled as vendor/artisans.
I'm sure there is more, but these are the main points.