I also posted this on the elevator pitch thread, but perhaps it's more relevant here.
Although this point has already been mentioned, I'll second it. Although, instead of saying gamification, which is accurate but doesn't really convey why the technique is so effective here, I would say it all comes down to this:
Reputation
If you don't have reputation management built directly into the fabric of a forum, the forum inevitably produces increasing amounts of white noise until finally it becomes a bloated source of primarily irrelevant content that slows you down from finding what you're looking for.
Reputation is not just for the individual star responders who know every hack in the book, but on a completely granular level where every element has its own reputation from the moment it's born. Each asked question faces the scrutiny of the community, to the betterment of the quality of the community, and thus discourages the lazy and unresearched questions from being asked in the first place.
Yes, MVPs have been answering questions for years over at Answers, but they've also been sifting through piles of carelessly asked and increasingly redundant questions causing the overall value of the knowledge store to suffer tremendously. Frankly, if salesforce Answers had a built-in reputation engine like the Stack Exchange from the beginning, being a self-cleaning oven as a result, it would still be my go-to place for finding and providing quality information.
So... my long-winded 2-cents for the elevator pitch is that the differentiating concept of a reputation-based, self-cleaning oven forum be included in the message somehow.
