Thanks for fleshing out the points you made in the blog comment.
I'm sure there are lots of secondary Moodlers out there who are wrestling with this same issue: how to transfer the energy and spontenaity of a message board (so beloved by our teenagers on social networking sites) across into discussion forums that also foster more discursive/detailed/reflective contributions?
But I also know that veteran Moodlers would say that 'rules' as such aren't the answer - it's about establishing the right culture and attitudes among students.
I certainly agree with the 'culture' principle. Yet I'm sure we can all agree that establishing this culture takes time, especially when a range of colleagues adopt different standards because they themselves are getting to grips with using forums (and wikis etc).
So our shared desire to find an easy way to display "rules" on forums doesn't undermine the culture principle.
It just reflects our practical need to establish expectations upfront on forums - perhaps in the way your mock-up suggests - and without having to add the 'rules' manually each time.
It's really about how best to foster good habits, rather than trying to stop bad behaviour.