Matt's article on Approachability, I wanted to add a few additional thoughts.
Approachability can be evaluated in many different ways, but somehow each measuring stick is as intuitive as the conceptual idea behind it. When instructing a game, how often do you receive befuddled looks, or a person interjects with a question as there is no simple interpretation?
The idea of approachability is visible in teaching games. Some of the greatest games have the simplest rules, allowing new players to find their bearings and head in the right direction on their first play. What about the games where everyone needs a practice round? A practice game? In searching for the next great innovation designers often lose sight of approachability as it fades into the horizon.
I'm of the school of thought that Chess cannot be improved upon and I'm certainly not alone. But if it was designed today someone might try to make it more complex than necessary, losing the beauty of it. What if when the black pieces end their move on a darker square on the chess board they gained a special ability? What if when the white pieces entered a lighter square on the board the player draws a card. This sounds absurd but it is the direction some designers are moving and it may not be forward. Designers are increasing complexity and often by relation there is an increase in strategy, but the deep end of the pool is quickly becoming a "no diving" zone.
Is it possible to achieve perfection on a the scale of approachability? Should beginners have a reasonable shot at winning? Should a game be so intuitive that there are no technical questions such as "Can I do this?"
I think the answer to all of these questions is simply "no," as approachability does not extend into the learning curve of a game. Approachability is simply the hike up to base camp, and it says nothing about the height of the mountain or the rate of incline. Beginners don't need a reasonable chance to win, and we all learn differently - rarely do we see a set of rules that is comprehensive enough that we don't even try to break from constraints.
Approachability should be the gentle ascent preceding the real challenge, not phase one of Mount Everest.