Let us assume you were to create a platform such as StackOverflow. It needs to be functional, fast and scalable, just like SO, in order to be successful.
What are the major challenges you would face in such a task? I'm looking for technical challenges rather than problems such as how to advertise it and stuff like that.
(I'm not looking for ways of creating a new site to challenge SO; I'm just interested in how difficult it would be to create it if it didn't already exist.)