Halo: Reach was one of the big game releases of 2010, so I was pleased to see a detailed technical interview with some of the developers on Eurogamer‘s Digital Foundry website. I recommend you read the whole thing, but I’ll summarize some of the notable rendering tidbits (the interview also covered multiplayer, AI, and animation):

• The previous two games (Halo 3 and Halo 3: ODST) used a “semi-deferred” approach, not for deferred lighting or shading but for decals. It sounds like they rendered a cut-down g-buffer (probably just diffuse color) in the first geometry pass (skipping small decoration objects like grass and pebbles), then blended decals into this buffer, and finally rendered the geometry a second time to do the lighting. Halo: Reach changed to a deferred lighting approach. Some lights were deferred and some weren’t, objects without decals or deferred lighting were only rendered once (this “hybrid deferred lighting” sounds similar to what Naughty Dog used for the Uncharted series).
• Halo 3 used spherical harmonics in lightmaps to store directional lighting information (detailed in a GDC 2008 talk, as well as a SIGGRAPH 2008 course – see slides and course notes). For Halo: Reach, Bungie developed an improved light map representation that gave them “the same support for area light sources, improved contrast, fewer artifacts, a smaller memory footprint and much better performance”. This sounds really interesting; I hope they will describe this further in a conference presentation or article.
• They developed a particle system which performs scene collisions on the GPU, using the depth and normal buffers as an approximate scene description. it can do tens of thousands of collisions / bounces per frame in 0.3 milliseconds (their previous CPU-based colliding particle system had a budget of 7 collisions per frame!). This system will be presented at GDC 2011 (the presentation will also discuss optimizations to their atmospheric effects system).  This is a great idea – techniques like SSAO use depth/normal buffers as approximate scene descriptions for rendering, but this is the first time I have heard of this being done for simulation.
• Halo 3 used two 8-bit-per-channel frame buffers with different exposure values for HDR effects (primarily bloom). Bungie described this scheme in a presentation called “HDR the Bungie Way” at two Gamefest conferences: USA in 2006 and Europe in 2007 – the 2006 (giant) zip file also contains an audio recording, but the 2007 one has more updated slides (including screenshots). The GDC 2008 talk mentioned above also discusses this scheme briefly towards the end. In contrast, Halo: Reach uses a single 7e3 buffer; this yields higher performance and frees up more EDRAM for shadow buffers but has less dynamic range (the primary result of this is loss of color in some bloom regions).
• Instead of MSAA, Halo: Reach uses a simple temporal anti-aliasing method. The camera is offset by a half-pixel in alternate frames, and the last two frames are selectively blended (the blending is turned off on pixels that have moved significantly since the last frame).
• They developed a new LOD system (to be presented at GDC 2011) which automatically generates low-cost models to be used far from the camera. Combined with improved occlusion culling and GPU occlusion queries, this enabled a significant increase in draw distance.