
Halo 3 was, in a way, doomed before it was released. No matter how well it did, people are going to judge its "wow factor" against how amazing the original Halo felt to play back when it first came out. Now that we are used to the weapons, races, environments and plotline, nothing is as cool as it was that first time. It's the same issue that plagues any series, be it Dune, the Matrix, Star Wars, etc.
Halo 3 does get a boost in that it is released for the 360, so you get an automatic upgrade in abilities there. We're playing on a high end HD TV, and yes, it looks really nice. The backgrounds look like a painting in motion, with swirling clouds and dancing dust storms. From the light glinting off the metal surfaces to the detailed shadows in the rocks, it can be really gorgeous. Is every detail as beautiful as every detail in, say Gears of War? Maybe not. Different developers focus on different items. There are always trade-offs for speed and visual appeal reasons. For a Halo world, I think they did a good job.