Say what you want about overzealous moderators, it is an incredibly useful resource. It's easy to take it for granted but I remember the days before SO and it was much harder to find good answers to programming questions.

In theory it could be replaced with something else but good moderation and a very product-oriented leadership (mostly programmers, not MBA types) is what made SO what it is. These intangible "soft" qualities are the hard part - it would be easy to duplicate its technology.