Mark "Have Blue" Levin of Freeverse has written up a Gamasutra postmortem of their port of Marathon 2: Durandal to the Xbox 360.

What's revealed is that the Xbox version isn't so much a port as it was almost a complete from-the-ground-up rewrite:

Going beyond a port. Without an emulator, huge chunks of the game had to be discarded. The original rendering code of the game was very well suited to its original platform, but was entirely unusable on the Xbox 360. MacOS’s proprietary graphics API, QuickDraw, was used extensively throughout; some of its features, such as drawing 2D lines, were absent from the 360 entirely. Screen-space arithmetic was done using fixed-point integer math, a crucial optimization a decade ago but a source of needless complexity today.

Texture mapping was done using blocks of PowerPC and 68K assembler -- no one on the team was able to directly read them. The entire renderer had to be rewritten in DirectX. As a result, the 360 version of Marathon 2 outputs a native 720p image at 60 frames per second, rather than the 480p30 of the original. And unlike most other classic ports, it uses texture filtering, true 32-bit rendering, and every pixel of a widescreen display.

The "HD" mode of the graphics, which in other games commonly refers to a post-processing filter, is a completely new set of images. All of the nearly three thousand sprites in the game were redrawn in true 32-bit color, allowing us to put our own spin on the look of the game.

Wow. That's an awful lot of love heaped on a decade-old game; but Marathon deserves it. Check out the entire postmortem for the story of what went right, and what went wrong, bringing Marathon to the Xbox 360.