What SHOULDN'T Django's admin interface be used for?
Generally, you shouldn't use the admin for access by people you don't really trust. Even though there's plenty of flexibility in terms of locking things down and controlling access (much more so since Django 1.0), the admin is still designed on the assumption that the people using it are trusted members of your staff.
I've been applying Django's automatic administration capabilities to some applications who had previously been very difficult to administer.  I'm thinking of a lot of ways to apply it to other applications we use (including using it to replace some internal apps altogether).  Before I go overboard though, is there anything in particular I shouldn't use it for?