Deus Ex is a linear game.
Anyone who's played it is calling me an idiot right about now. Linear? DX is the antithesis of linear. It's always up there with Fallout or Arcanum as the pinnacle of player choice. But in reality, that's all a complex illusion. The game only really offers a choice a handful of times, and other than the ending (there are three options there), the effects are always minor. A character may show up again, you might get a different reward later on. When it comes to big decisions, you don't have a choice. The order of levels is predetermined—except in Hong Kong—and you can't choose sides.
The fact that Deus Ex is linear but absolutely no one who plays it thinks of it that way is the master stroke of the game. It is a brilliant piece of design work that many games have since tried to replicate (Bioshock is a good recent example), but not one of them has succeeded.
The trick is that, while the game progresses from level 1 to level 2 to level 3, no matter what, within each of those levels you have many choices. In any given situation Deus Ex will give you at least two ways to proceed, and usually more than that—if you even end up in the situation. On a single playthrough, it's unlikely you'll see more than half the game. I still find new things every time I play, and I've been at it for ten years now.
For example, there's a level where you have to get to a ship. You're dropped at the gates to the Brooklyn Naval Yards and you have to get through it. During the course of this level you can hack computers, pick locks, sneak, or just shoot everything. You can fight the guard mechs or turn them against each other. You can go through a tunnel network beneath and avoid the whole battle. If you can't hack, you can get security codes from the workers. There are three different factions at work—the enemy, the friendly soldiers, and the neutral dockworkers. Once you're inside (which you can do by going through ducts or through the security office, which either has a friendly soldier or you have to fight), you can sneak aboard the ship, go up the ramp killing everything in your path, or use a crane to hop over to the superstructure. And that's not to mention all the different methods of going about this, depending on how you've customized your skills, augmentations, equipment...
And the entire game is like this.
There are so many things designers can take from Deus Ex, but if they only take one, it should be this. Don't railroad your players. Not every game needs to be nonlinear; not every linear game has to be on rails.
Beginning to think like us now, are you? Be careful. Paranoia is a drug; you can get addicted.
Deus Ex' storyline is an amalgamation of every conspiracy theory you've ever heard, plus a few more. It quickly becomes apparent that everything you've been told is wrong, the world doesn't function the way you were led to believe, and you can't trust anybody.
And it works. To this day there are a few characters I can't figure out. The writing is just subtle enough, their dialogue just ambiguous enough that it's impossible to tell whose side they're really on. The game never talks down to you, and it doesn't explain anything unless it's reasonable that you'd get an explanation. When you get a funny feeling from your ally, unsure of whether he's benevolent or just as bad as your enemy, that feeling isn't resolved. You don't get to see into his head, nor do you have some clumsily-inserted diary or monologue later. You have to take your incomplete information and decide whether or not you trust him.
The game manages to maintain this throughout. There are a few characters who are clearly on one side or the other (though the game carefully maintains sympathy for the villains, and you're never quite sure if the villains are wrong), but for the most part even your most stalwart allies are questionable. Games often fail when they try to maintain a feeling throughout—horror games are notorious for this. System Shock 2 manages to stay terrifying the entire time; few others do. It's difficult to keep evoking the same emotion in the audience again and again without losing effectiveness, but Deus Ex manages it with paranoia.
As a history and politics person, the depth of thought behind game is one of my favorite parts. Next post will go into how Deus Ex used literature and political philosophy to elevate itself above the competition.