Viral player stories. When you create games with deep systems, player run into amazing, emergent scenarios on a regular basis. From these moments of player experience grow myths and legends. Players tell them. Press repeats them. Your game goes viral without requiring any of the whirring, queasy machinations of your local social network dealer.
Why does this occur? It happens without prompting. It requires no points, no bribes. It is as if, after the right experience, the urge to tell stories bubbles up innately from inside even the least imaginative human.
A story, at an evolutionary level, is a lesson in success or failure intended to improve the survival rate of the tribe. This is why we create them. This is why we share them. This is why we consume them. Like play, story (even fiction) serves a function. Good stories were at one point a matter of life, death and reproduction. Humans have a nose for truths and when we spot them amidst the maelstrom of daily experience, we instinctively share them.
When we, as game designers, create meaningful systems whose depths are only revealed after a process of deep mastery, players instinctively extract stories from their experiences within these playscapes and pass them on to their friends and family. "When I kicked the soccer ball, my foot slide on the wet grass and I heard a distant sickening crunch in my knee." To experience a unique lesson that you've learned in your bones after a thousand trials is to hold a treasure. And being human, we can't help but share. Over and over and over again.
A critical realization for a game designer is that meaningful success and failure, the basis of stories, can only exist in the context of the systems and value structures we design. A gripping tale of trust crushed in a game such as Eve exists because a designer made a very explicit set of rules that defined the concept of economic value and politics and trust. Remove the value structures inherent in the design and the stories go away.
One sign of a great game is therefore not the story that the designer tells, but instead one that contains mechanics robust enough to yield player experiences rife with lessons that must be shared. As an exercise, look at the mechanic (apples, trees, 9.8m/s^2) and the story of gravity (Apple falls on Newton's head) as two distinctly separate elements. The designer's role is not to tell the story of Newton and the apple. Players will perform that service just fine. Instead, our unique role in the process is to define and polish the system of gravity.
Don't build games in order to tell a single story. Build meaningful systems that create an explosion of culture, spread by the players who are absolutely thrilled to share what they've learned.
Nethack, Populous, Lemmings, Sims, SimCity, Minecraft, Spelunky, Fantastic Contraption, Dwarf Fortress, Team Fortress, Ultima Online, Civilization and some I've forgotten. Any others that you feel compelled to share?