I'm a teacher, and I'm using Drupal to manage my course web site. I've been using it for a couple years now, and in general it has worked really well. There's just one feature that I really, really want and have so far not been able to set up practically: private forums for each student.
I've successfully used taxonomy_access to control who can see a section of the forums. For example, I've established roles called Fall2006, Spring2007, Fall2007, and soon Spring2008. Then I can assign students to those roles as they sign up, and set up permissions so that each class can access its own forums but no one else's. That's fine. But what I'd really like is to set up a private forum for each student. So in a class of twenty students, there would be twenty forums, one for each student; I would be able to access all of them, but individual students would only be able to access their own.
Theoretically, this is possible to set up using taxonomy_access. But I would have to set up a separate role for every single student, and assign permissions for every forum - I ran the numbers at one point and it works out to several thousand configuration changes to set up private forums for twenty students. I don't have that kind of time, and even if I did, chances are good that I would make mistakes in the configuration and wind up in permissions hell all term.