Gamer Gaia has put together a review of Ion Storm's masterpiece and its sequel where they analyze what made Deus Ex so special and loved by fans to these days and why Deus Ex: Invisible War was not as well received. They end up awarding the whole package a 9.
Deus Ex is a FPS game with heavy RPG elements involved. You are playing the role of a newbie UNATCO agent, named JC Denton, and you are fresh out of the boot camp when they assign you for a peace keeping operation at the Statue of Liberty. Unlike standard agents though, you have nano-augmentations and you are one of the few hi-tech agents the government has, like your brother Paul.
Now, I will try to avoid spoilers as there may be people who want to explore this gem and its beauties (and story is everything in Deus Ex). Throughout the story, you will face with many twists and turns, meet with many memorable characters, discover lots of conspiracies and conspiracy theories linking to today's real life events, realize lots of (potential) future references that is coming true day by day (when this game was out, most of those references were fantasies but now, you can see a path towards what the game had suggested a decade ago).
This is why Deus Ex was brilliant in means of story telling. This is why Deus Ex was awesome in means of roleplaying. Deus Ex had so many beauties in it, like cause and consequence scenarios. Except for a few sequenced battles, you were able to avoid most of the combat. In one game, you were able to kill an important character at point A and change the course of many little things where as in another play through, if you had decided to let that character live, things would shape differently and you may have had other confrontations. I remember discussing the game over ICQ (MSN wasn't even useful back then) with friends and more than one time, we managed to shock each other like (WHA?! You can avoid that battle?) or (WHA? Can he be saved?!).