I finished The Design of Everyday Things recently, and while it does not deal directly with games, I'd still consider it a must-read for anyone interested in game design. Norman has some great insight into what makes an artifact usable and one area where games often suffer is usability, both in the gameplay itself and in the game's support systems. The downside of reading DOET is you'll start seeing bad design everywhere and those around you will likely tire of, "Gah, look how poorly designed that door is! If a door needs a 'pull' sign, it's not well designed!"
One area of Norman's discussion that I found particularly applicable to games was the notion of width and depth within interactions. A wide but shallow interaction affords a small number decisions that have a lot of possibilities, but are not sequential or meaningfully connected. A narrow but deep interaction has a large number of decisions, but the possibilities for each decision are relatively constrained. The culinary analogy he provides is that buying ice cream is wide but shallow- there are a lot of options, but once you've decided on the flavour, there are only a few simpe decisions (cone, toppings) to be made. A nine course dinner is narrow but deep; at each course, there are only two or three possibilities, but there are a many sequential decisions to be made.
Most of our daily decisions are either shallow or narrow, which is why most of our reactional activities are both wide and deep. Amongst games, width and depth can, and should, also be varied to affect the game's interaction. "Shallow" is often used pejoratively as a gameplay descriptor, but here, I'm using it as Norman does, to refer to the size and sequentiality of the decision space.
The width and depth of an interaction are not binary states, but rather a continuum. It's important to consider both aspects of interaction, and deliberately design toward certain levels of width/depth that best support the intended experience.
A game like Animal Crossing is wide but shallow. There is a lot to do in one's town, but the actions themselves are nonsequential. This allows for very freeform play that's quite accessible. Left 4 Dead is narrow but deep. At any moment there are relatively few decisions (auto shotgun or assault rifle? Heal myself or an ally?), but each decision sequentially affects future decisions. Sins of a Solar Empire is both wide and deep, with a myriad of decisions that will directly affect the next decisions made.
It's possible to look at the local gameplay and global gameplay separately. Far Cry 2 offers relatively narrow gameplay decisions at any given moment, but at a higher level the global decisions are wide and nonsequential. Most MMOs a la World of Warcraft start with a narrow and shallow local and global decision space that becomes wider and deeper.
Width and depth are descriptors, not advantages. Trying to make a game seem artificially deep will often obfuscate the things about it that are most interesting and engaging. Adding unnecessary decisions to a narrow game unnecessarily will often dilute the meaning of those decisions and create a paradox of choice, where the player is more worried about the things they won't experience than the things they will as a consequence of their decision.
The Design of Everyday Things stresses being deliberate and conscientious about design. The width and depth of gameplay decisions should be considered carefully, their ramifications understood. Doing so can help keep the design focused and prevent wasting time on features that will only add needless complexity.
Do you prefer a game to be narrow or wide? Shallow or deep? Are there other ways these characteristics can inform the way we look at games? I'm curious to hear your thoughts.