What is Deus Ex?
I'll try to be brief. (Those of you who know me know I'll probably fail…)
Fictionally, Deus Ex is set in a near-future version of the real world (as it exists if conspiracy buffs are right). For some real shorthand, call it "James Bond meets The X-Files." (Remember that seemingly innocent claim that Deus Ex is set in the real world. It'll come up again shortly…)
Conceptually, Deus Ex is a genre-busting game (which really endeared us to the marketing guys) -- part immersive simulation, part role-playing game, part first-person shooter, part adventure game.
It's an immersive simulation game in that you are made to feel you're actually in the game world with as little as possible getting in the way of the experience of "being there." Ideally, nothing reminds you that you're just playing a game -- not interface, not your character's back-story or capabilities, not game systems, nothing. It's all about how you interact with a relatively complex environment in ways that you find interesting (rather than in ways the developers think are interesting), and in ways that move you closer to accomplishing your goals (not the developers' goals).
It's a role-playing game in that you play a role and make character development choices that ensure that you end up with a unique alter ego. You make your way through a variety of minute-to-minute gameplay experiences (which add up to a story) in a manner that grows naturally out of the unique aspects of your character. Every game system is designed to differentiate one player-character from another, and to allow players to make decisions that reflect their own biases and express character differences in obvious ways in the game world.
It's a first-person shooter because the action unfolds in real time, seen through the virtual eyes of your alter ego in the game world. Your reflexes and skill play an important part in determining your success in combat. However, unlike the typical FPS, Deus Ex doesn't force you to shoot every virtual thing that moves. Also unlike the average FPS, in which gameplay is limited to pulling a virtual trigger, finding blue keys to open blue doors and jumping to reach seemingly inaccessible locations, Deus Ex offers players a wide range of gameplay options.
And finally, Deus Ex is like adventure games in that it's story-driven, linear in narrative structure, and involves character interaction and item accumulation to advance the plot. However, unlike most adventure games (in which you spend the bulk of your time solving clever puzzles in a search for the next static, but very pretty, screen), Deus Ex asks players to determine how they will solve game problems and forces them to deal with the consequences of their choices.
Deus Ex was designed from the start to combine elements of all of these genres. But more important than any genre classification, the game was conceived with the idea that we'd accept players as our collaborators, that we'd put power back in their hands, ask them to make choices, and let them deal with the consequences of those choices. It was designed, from the start, as a game about player expression, not about how clever we were as designers, programmers, artists, or storytellers. Which leads naturally to a discussion of having clear goals -- the first thing I think we did right.