Revolutionary to the FPS genre, invented major mainstays for multiplayer and campaigns such as two weapons, recharging shields and shooting creatures different from you.
It isn't perfect however, the missions “Library” all the way to “Keyes” are a certified nightmare grind, involving repeating map design, some of which involves backtracking through the most boring corridors and rooms, and the most brutally difficult and annoying swarm enemy possibly in all video-games.
However despite this, I couldn’t stop playing it, despite how frustrating and how much I wanted to strangle those explosive flood enemies I wanted to keep trying. So I can easily say I understand why this game is so iconic.

Also goes without saying some of the best music out there.