I think the biggest challenge is to draw in non-technical people from outside SOFU - the chances for this are very, very good though, and the minimalistic UI will help a lot. However, a lot of things that are self-explaining to a tech-savvy thirty-something are not so for our moms, grandfathers and aunties. I bet many of them have not heard the term "faq" before.

The second biggest challenge will be to achieve a "modus operandi" - a standard for what are acceptable questions and what are not - that is as mature and refined as the one on Stack Overflow. This standard will differ slightly for *every* site due to the specific nature of its topic. 

I'm interested to see how this plays out.