I think that the diffuclt of stackoverflow is to get community (very good quality community, not like yahoo answers).
Not only that, also use cases from stack overflow are pretty cool and adapt very well to get a good community.
About work-power a good skilled programmer could start it, if at full time like a month or less could do it. BUT! the programmer should have the idea,not a freelance or something like that, freelance or slave monkey coder could take a more time to do it.
But there are more problems, like money to invest at very begin of the app for example in hosting / server power costs.
Also stack overflow, could be compared to forums...its like a forum evoled or something similar.
Someone said that requires a lot of work power, I disagree if you start something to get the best scability,etcs (like project of big scale) you are going to death of that project.
Start something simple, very simple when there are scability problems start with that but no at begin!