If only it were that simple.
Firstly, the average forum administrator is a data controller. Sucks, but it's true. So they have responsibilities simply by running a forum and collecting email addresses.
The registration agreement probably needs a rewrite, but whether it does or it doesn't... you need to track when people agreed to the terms. You also need to check when the terms change that people are still happy with the terms, and track every subsequent acceptance.
There's also the fact that users can request a copy of all the data you hold on them, electronically in a open format. (Ideally, provide this in a self-service format.)
And permission to delete account pretty much needs to be given to everyone, it must be actioned inside a month, and all data that is personally identifiable needs to be able to be removed. As a minimum this would imply users that chose to use their name as their username would have to be scrubbed before deletion was approved. This would also need to scrub the database of all email addresses stored in the messages table against a user's posts.
I'm also not clear on how this would work for guest posting, or whether this would be mandatory to be disabled for the sake of being able to prove consent to data collection.