When a Category has a Mandatory Property which has a Validation Rule defined as Dropdown and with a default value, when creating a ticket using that Category, the default value is not set and it is necessary to be manually defined.
Example on how to reproduce the behavior:
From R12 release only on, this behavior changed: it's been decided to change the way defaults for properties work.
The behavior exhibited is working correctly.
Dropdown will show "<empty>" on the User Interface. This will enable user to really enter some value when the property is set as "Required".
So, if you want the property to be set as "Required", do not set any default values to the property. This will enable users to enter some value to the properties.
Or, if you want the property to be set to some default value when creating a ticket, then do not set the property associated with the Request Area or Category as "Required".