Enebias: As someone who doesn't know the game, I ask: what makes it so great, in your opinion? It seems a common shooter from an external point of view, but since everyone regards it like one of the best games ever there must be something special...
Disc combat is fairly unique to the game among FPSs. There are other, more standard weapons to use, but mostly I just use the disc when I play. Feels good to block a shot with it and then hurl it at an enemy.
The graphics hold up extremely well because how abstracted they are. I actually think the game captures the aesthetic of the original movie better than the Tron Legacy movie that came out a few years back. The game also has a better story than that movie, too. It's clever how it uses the computer world as inspiration, such as a computer virus turning the people into diseased maniacs, or a militaristic villain being called the Kernel (if you groan at this stuff, you probably won't like the movie, either).
The power-up system is interesting, I think. You can't just load up everything at once because you only have limited space to work with, and that space gets even more limited if you're infected by the virus and your systems start being corrupted.
It's just a well-made game all around. Do you know the No One Lives Forever games? If so, think of this game as a sort of spiritual extension of those games. Same developer working in a similar style but with a different theme.