the installation of such system is like 1 hour of work.
In my point of view something like stack overflow is the best but i'm not sure if it's open source.
Setup time doesn't matter, at all. It's on-going maintenance and "everyone has better things to do" that means there won't be a forum when the mailing lists work fine. Setting up a forum means you volunteer to keep it running -- forever. You can't just pack up and shut it down in 2 or 3 years when you get bored with it.
