Though I'm looking forward to Halo 4 (Big Halo fan here), I do understand why Bungie had to drop the resolution and AA to develop the game engine they did for Halo 3 and Reach. That Halo look requires a lot of alpha transparencies, post process effects with HDR and MDR lighting and the levels need to be large to accommodate the vehicles and that sanboxy type firefights. The 360 Halo games all fit within the 360 10MB EDRAM framebuffer limit without tiling it out to the main ram (which uses up precious CPU resources) so they have tons of tons of bandwidth to play with.
If 343 decide to go native 720p with AA then something has to give. Less use of heavy alpha, less bad guys to fight at any one time, cut back on HDR, smaller level design, tile out the framebuffer to the main ram....must be something they cut back on. Maybe they could use FXAA.
