phpBB should add in unsubscribe functionality to it's mass email, as well as email notifications to allow users to easily unsubscribe to such messages without having to login and follow a complex process in the UCP. This is not only convenient, but in some cases required by law in some jurisdictions, especially if an admin is sending "marketing" like emails from the ACP.
It would also be beneficial to us Canadian's to have some sort of "CASL compliance" feature (similar how there is a COPPA feature) that would give administrators the option to show a selection box on the registration page so the user can decide to opt in (not opt out as required by CASL) to mass emails, and while not required by CASL, an opt in for notifications emails as well. It could go along with Pony's suggestion here with regards to having a "Legal" section in the ACP where all these features could be grouped.