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!
This was way better than Halo 3 in these aspects:
1. Story actually made sense
2. Multiplayer had some lag, but not too much that it affected your gameplay
3. Honestly, the story is more thrilling in Halo: Combat Evolved, especially when meeting The Flood.
THE original unique shooter, one of my first, the fun days of playing on two controllers against one another are one of my fondest memories of this game, an action packed one at that, gripping and VERY interesting campaign which first got me hooked on the Halo Universe made itself out to be one of epic proportions.