Aha... so THIS is why I can't see any forum descriptions as a new user! As a new user, my account defaulted to the trending topics homepage. I kept trying to access the private forums, but since there's no way for me to see the forum description saying 'please submit your staff #' etc., I just got a totally generic 'access denied' error message.
Of course when I posted about this, here, in the 'problems or queries' board, lovely veteran members gave me a warm welcome by just assuming I couldn't read...
A review of what onboarding looks like for new users wouldn't do any harm.