But what makes it great?
Depth.
With a hugely deep story, where the world is just about in ruins makes for a rollercoaster plotline suitable for any movie or block-buster sci-fi cyber-punk novel. In fact it's so involving that you may start thinking that you are JC Denton, the half-man half machine hero.
This is somewhat due to how much you can influence the style of the game. By always being short and annoying in your responses to people (being the bad-cop if you like), people will in turn be nasty to you. Cap too many people and your bosses will start limiting your bullet supply. Who JC actually is is up to you. And you've got loads of time to develop. With between 20 and 40 hours of game in front of you, which believe me, you will be more than happy to play, Deus Ex is a massive hitter. And that's before you take into account that Deus Ex is a multi-route quasi-multi-linear game. In other words your decisions mean a lot, and that what you decide to do or who to shoot etc. will change how the game continues
Though what Deus Ex is most recognised for is the great mix of genres; stealth, action, role-playing, adventure, detection, first person shooting, sword combat. It's all in there.
So all that we will say is play it, you will not regret it.