"The visual design, the interaction design, the writing and reading experiences, the social incentives, the notification system: all of it is top notch."

I SO disagree here. It's a mess. It's 10x more complex than it needs to be. And it's horribly unapproachable for new/non-geeky users. I've had non-technical relatives sit down and try to use it who were positively baffled by the user-experience.

I think Quora is an example of what happens if you listen exclusively to your most active users, when you should be tempering that with conversations with new and drive-by users. The Curse of Knowledge is deadly for product designers (http://hbr.org/2006/12/the-curse-of-knowledge/ar/1).