HALO is one FPS that really did introduce many new gameplay elements. It offers all that is said below and the modifications already in development look to refine and improve the "HALO experience" even more!
Halo Combat Evolved is a good game. But its not amazing.
2 gun limit. Bullet-sponge enemies. A lack of variation in the enemies. The variety of weapons are a bit interesting but is held back by the fact that you can only carry 2 at a time.
And sometimes the shooting gets tedious. And tiresome.
Mister Chef and Cortana are good and likable.
Cortana's voice acting is very good.
Halo "Custom Edition" is where the fun is at. A bunch of maps to download. The modding community has really kept this game alive.