Halo added a lot of game play features like 3D character 1st person view, in a 3D world. It added jumping, fast paced game play. This changed the way players played it, because they could move freely around a world with fast paced combat in real time looking at a beautifully crafted and strategically designed map. Also AI was made a lot smarter, because it would interactively engage you in combat. The features that were added/changed to make the FPS into what it looks like today were the game play, the graphics, the AI, the maps, and the mechanics.