Also called “Theme First” or “Experience First” design, this methodology is concerned with identifying a specific feeling or aesthetic experience a designer wishes to capture and breaking it down into its key verbs and scenarios. From there the process becomes a matter of translating those verbs and scenarios into analogous Game Design Patterns and gradually Merging Gameplay Mechanics until you arrive at an appropriate rules construct.
The risk with top down design is filling the game with incredibly generic mechanics by exclusively pulling patterns from games with a similar theme, effectively becoming a reskin of games the player has seen dozens of times before.
Raph Koster gives the advise of trying to model an unusual event by telling a story that isn’t often told or by adopting unique gameplay metaphors.
The opposite of Bottom Up Game Design