1. Optimized CLUTs for wall textures and other graphics. This gives the illusion of textures with far more color depth than they actually have. For example a set of textures containing a lot of dull brown-colored walls does not have to contain bright colors in its color look-up table. Instead, there is room left over for a wider variety of brown shades.
2. More frames of animation for each character. Up to nine frames for any given action, and hundreds of frames for each character.
3. In-game music in addition to ambient sounds
4. Swinging doors and smashing windows. Through extensive use of Forge, Bungie's level editor, I have developed methods of approximating these complex events.
5. Enhancements to the Marathon 2 source code allow for many more significant changes.