Mission statement

While a fairly self explanatory term, the mission statement is used slightly differently in a game's design. According to Wikipedia:
"A mission statement is a formal, short, written statement of the purpose of a company or organization. The mission statement should guide the actions of the organization, spell out its overall goal, provide a sense of direction, and guide decision-making."

Obviously, we'd want to replace "organization" by "game". A game's mission statement should describe the intended high level user experience, what the player should feel and/or think, the spirit of the game. It usually doesn't describe the game itself, as the mission statement is (or should be) written even before the design process starts. Let's have a look at a few examples:

• Assassin's Creed: "You're a blade in the crowd"
• Fable III: "Your Hero, Your Adventure, Your Kingdom"
• God of War: "Anger and Chaos"

Those examples quite fit what comes out of the corresponding games. In the first Assassin's Creed, that mission statement is the name an achievement whose requirement is "Kill one of your main targets like a true assassin". In Assassin's Creed 2, it's the name of an assignment.

The mission statement has a very important role, as outlined in its definition: "The mission statement should [...] provide a sense of direction, and guide decision-making.". In game development, this has a quite simple implication: whenever you want to add a new feature that sounds cool (be it a game mechanic, an animation system, a special sequence, etc.), look at your mission statement. If that new feature doesn't participate in any way, shape or form to the mission statement, rework it, or simply throw it away, even if it sounded cool.

An example: In Assassin's Creed, it could have been nice to talk to the villagers to gather information. However, that's not what a stealthy assassin (a blade in the crowd) would do. Instead, overhearing conversations is much more in the spirit.

Of course, this all means you need to get your statement right as early as possible (if not from the beginning), which isn't supposed to be easy. It's part of the things that make a good lead designer (yeah, that should be the lead designer or the creative director's job) and comes with experience.