It'll take a while to explain. And hell, it's almost the tenth anniversary; let's give the best game of all time some well-deserved attention.
Relevance and Worldbuilding
Deus Ex was released in 2000, yet the World Trade Center isn't in its depiction of New York. Part of the skyline had to be removed because of memory limitations, so the WTC was chosen to go. Their explanation: it was destroyed by a terrorist attack.
The game's storyline revolves around terrorism. It's the year 2052, and after a major terrorist attack in New York City, the United States is descending into fascism, largely as a response to terrorism. The country is falling apart—much of the west coast has been destroyed in a massive earthquake, there has been a low-level civil war for several years, the economy is collapsing, plague is rampant, and there is a war with invading Russian forces in Texas. People are encouraged to report their neighbors, electronic communication is all monitored, anti-terrorism forces have free reign, and the gap between rich and poor is so great that each side is hardly aware of the other. In this nightmare future, gasoline prices can be as high as $4 a gallon.
The developers themselves have since commented on the real-world parallels in the game, often mentioning that it creeps them out a bit. You can access public terminal and read terrorism alerts that would be right at home coming out of the DHS.
Some of these are just tropes of the genre. Deus Ex is one of the few cyberpunk games, and a world in chaos, transitioning to something new, with governments in decline and corporations taking over is a standard setting. But playing it in 2000 and again a few years later were very different experiences, all because of the real-world relevance the game had since gained.
All of that may seem irrelevant to why the game is good, but it's not. It shows the depth of thought that Warren Spector and his team put into the worldbuilding that the game is based on. The game goes into conspiracy theories and future technology, but they still wanted it to feel real. It had to—it's set in the near future of the real world. Without introducing magic or an apocalypse, they had to ground their setting in something that would be futuristic enough to be 2052, but recognizable enough that the player could accept it. 52 years isn't all that long; things wouldn't change so much that they could make the setting completely alien.
So they took the real world (for the most part—the conspiracy storyline changes some of the motivations behind actual events), added a bunch of fictional events, and extrapolated the consequences while staying true to how things really work. Then when some of those fictional events became real ones, the real consequences eerily mirrored the fictional ones from the game.
Many other games, maybe most, don't take this sort of care with their content. They don't have to—there's plenty of room in the world for completely ridiculous games like Team Fortress 2, but even in those situations the good games pay attention to what they do. They don't just throw shit against the wall and haphazardly cram all of it into the game, unless you're Derek Smart.
Most importantly for Deus Ex, the game's authenticity makes you accept what's going on early. Later the conspiracies come out and the game starts throwing Art Bell's acid trips at you, but you accept it because it's all so well thought-out and grounded. You begin to think that maybe they have a point, maybe the Illuminati is manipulating global financial markets to bring about one world government...
Deus Ex always keeps you guessing. Even now, almost ten years later and more replays than I care to admit to, I can't pin it all down. I can't decide for sure who is right or wrong or if certain characters are trustworthy. And all of that is due to having one of the most strongly-constructed settings of any game.
But there's more... and more posts to come.