Some blurbs therefrom:
"Boasting 2,700 active users ranging from especially bright undergrads to Fields medalists, the basic function of the site is to answer the highly technical questions that crop up in math research."
"...organizationally, Math Overflow stands apart from its predecessors. Math Overflow is a community-moderated forum; users vote on the most accurate answers to the questions posed and gain reputation points based on participation, the most active of whom are granted various moderation privileges. The best answers are voted to the top of the page, while the worst ones are voted to the bottom."
"Math Oveflow is almost an anti-social network, focused solely on productively addressing the problems posed by its users. Heavily moderated, the guidelines for asking questions are designed to discourage unnecessary chatter and keep the community's focus on a question at hand...
"We've tried to make the forum as 'professional' as possible," said Scott Morrison..."
"Math Overflow has been a something of a revolution for how collaborative math is carried out on the Web..."