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 is an absolutely must-play sci-fi shooter and a masterpiece. Every aspect including the plot and gameplay of course is perfect. Replay value is no less great and I say immortal, and as for graphics being out-of-date, it's a minor "-", 'coz there's an anniversary edition for the picky ones. The game's an example of how sci-fi shooters should be made.