There is currently an option within each instance that "Subcategory is a mandatory field", which requires a subcategory when creating a new ticket. (Service Desk → Categories)
This is very limiting, as not all of our categories even HAVE subcategories. If we enable this, you can't submit a ticket within a category which doesn't have subcategories. (This, in itself, seems like an oversight in my opinion.)
It would be nice if I could decide for each category if I want to require the selection of a subcategory.