I have to admit I too struggled a bit with getting categories visible in the Feedback module. It is frustrating enabling this field, updating the module, then being totally confused as to why the categories aren't visible...no error/warning messages. And thanks to this dicussion thread, I was able to resolve the problem.
Now, I don't know what to suggest to make it intuitively obvious to the user as to how to resolve (make it work), maybe expanding a little on the help in the hover text on the "Send To" and "Category" field properties?? Maybe along the lines of:
Modified Send To (Hover Help):
"Enter the email address of the recipient of this message, leave blank if Category List Visible enabled."
Modified Category (Hover Help):
"Select the category this message belongs to (categories are created via the Feedback Lists) . Ensure at least one Category is selected if Category List Visible is enabled."
Jack