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.