* The graphics are stunning. At first glance, it doesn't look like too much of an improvement on Halo 2, but some of the lighting effects and details are beautiful. The jungles and some of the alien world levels are amazingly pretty, and the Flood levels are disturbing in a way the space zombies never managed before.
* I finished it on single-player Heroic on day one (it took about nine hours of play); I've started working through it on Legendary, and it's tough, but much more reasonable than the viciousness that was Legendary Halo2.
* Terminals! Like in Marathon!