Pros: Soundtrack, character development, storyline, setting, general atmosphere, decision-based gameplay, and much more
Cons: The voice acting is off at times, and the game hasn't aged terribly well from a technical standpoint, but these aren't even my personal complaints
A good game fulfills a player's wishes. A great game introduces a player to what they didn't realize they wanted. Deus Ex, in my eyes, is one of the greatest games ever made, and certainly the best on the PC. It brings a world to life in a way that few others have succeeded at. I highly recommend it to anyone who enjoys System Shock, Metal Gear, Thief, Omikron: The Nomad Soul, or anything similar. I must warn you though, you'll never get the soundtrack out of your head (as if you'd want to).