It being such a broad category is of huge revelance, because it means it does bascially nothing to inform design. Has RPG elements could mean that WoW should focus on its first person gunplay. Or its complex series of consequences from conversations with NPCs. Or the accuracy of its database of football players. Or its epic space battles.
Having RPG elements doesn't carry enough specificity to make design decisions for WoW. For any game.
It's part of a larger problem of how games are classified by their systems, rather than the core reason players play them.
To give an example from films - Pretty Woman, Blues Brothers, and Die Hard all have scenes with cars in them. We don't define them as "movies with cars", and go to the cinema to see the latest "films with cars in them". We look at romance, comedy, action.
If you made decisions as to what happened in any other medium based on the presence of a particular element, rather than choosing the elements based on the theme and purpose of the work as a whole, you would be laughed out of the room.