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!
Adequate shooter with its own style of playing. It has unnecessary difficulties that are worshiped by its fanboys, but once you stop listening to them you're still left asking why the heck so many issues continue to go unchecked from this game onward to the latest ones. The Halo 1 pistol is the least of your worries.