We know that product requirements should not contain design assumptions or design specifications. Following this principle has interesting consequences when the product is a game. Most games solve one problem: boredom or lack of entertainment. So is there merely a small set of requirements for such games that state in measurable terms how entertaining the game will be? If so, isn't it absurd to be so strict in separating the "what" from the "how"?
A few things:
First, it is true that the functional requirements for a game are likely to be sparse. Developing a game is unique in that it is largely a matter of design and creativity.
Second, a game will actually have many nonfunctional requirements. Things like scalability, availability, and usability all apply.
Third, any sparsity in the requirements specification for a game means the design specifications will be that much heftier. The developers will still know what to build; it's just that the designers arguably have a lot more responsibility in speccing out a game than in speccing out most other products.
Finally, the role of product management in game development is still vital. The product manager needs to identify and profile the right psychographic segment. Doing so enables the designers to design a game that is fun for that segment.