A user note feature would be very useful if it was part of a larger, centralized system, like the report center.
The user note function in vb had the major weakness that it was not centralized and thus there was no way for staff to keep track of user notes. No one is going to notice if you post a new user note to one profile, while a website may have thousands or even hundreds of thousands of profiles. It can take years before anyone encounters the user note.
I hope that XF will implement a much more useful way of handling staff user notes and staff user alerts.
Pretty much all websites have threads in their staff forum to post and discuss staff comments about specific users. Some sites use one thread for this, some sites use one thread for positive comments and one thread for negative comments, some sites use multiple threads (one per user)
A user note system should be centralized, so that staff can easily follow whats going on and even get alerts about important community matters.
I suggest that user notes should be visible within the report center or something similar to the report center. User notes should also be accessible from user profiles.
Staff should be able to reply to a user note, so matters can be discussed thoroughly.
It would be valuable to be able to include a severity level, so that all staff can be immediately alerted about issues with high severity, while only staff related to the issue/forum area need to be alerted about low severity issues.
There is quite a difference between a user note about a member repeatedly posting low content and a user note about a user being embroiled in a wide-scale forum riot or who leaves a suicide message.
Often user notes are related to warnings and support requests. Therefore user notes should be connected to the related warnings / reports.