I think a key item in Stack Overflow's success is that they can predict with high reliability how big it will get. They are safe going with scale up because the

I think a key item in Stack Overflow's success is that they can predict with high reliability how big it will get. They are safe going with scale up because they know they will not have to scale out. Compare this to a more mainstream consumer website, say FriendFeed, which is currently on roughly the same scale as Stack Overflow. FriendFeed's business model is more "swing for the bleachers" and success would mean scaling to the size of Twitter. They need an architecture that can handle fast growth if it comes. Stack Overflow is a much less risky proposition. They had excellent knowledge of the market, knew they could pull all the traffic from expertsexchange almost overnight, and they knew that scaling to tens of millions of monthly visitors was something they needed to worry about.