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!
Pistol and assault rifle are changed to halo 2 weapons. The ghost and shotgun fires needles. Plasma rifle shoots out plasma grenades. Scorpion machine gun fires sniper bullets. NOTE: this is for the original Halo: Combat Evolved NOT Custom edition. Back up before you install.