What SHOULDN'T Django's admin interface be used for?
User-specific privileges. I myself had been trying to work it into that-- some of the new (and at least at the time, undocumented) features (from newforms-admin) make it actually possible. Depending on how fine you want the control to be, though, you can end up getting very, very deep into the Django/admin internals. Just because you can doesn't mean you should-- it's easier and less fragile to do so with a custom admin app.
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?