I could not spot a real improvement from Halo 2, bar the hyper-realistic environments of Flood-infested walls, the addition of some vehicles, and the ability to dislodge heavy guns. It seems most effort went into just the environments while ignoring gameplay and a fresh and good storyline. It's quite a pity because the first Halo was such an epic achievement, while the second, while unable to replicate the first's grandeur, had quite a few innovations.