And this is why I think the sound design of Halo 3 is so ingenious: it uses sound, rather than vision, to expand your hegemony.
Let me explain. One of the big challenges when playing multiplayer first-person shooters is that it's essential to expand your spatial awareness beyond what's going on withinin the frame in front of you. Even when you get acclimated to the maps, and develop this ingrained lizard-brain consciousness that there is a wall behind you and to your left, you must understand where your enemies are in order to succeed. And this is possible when you learn to map the blips on your radar into your lizard-brain wall-consciousness. Once you have all this under you belt, there's still a last thing to consider, which is what weapon your opponent has. You have a split-second to gauge how you're going to approach this encounter-- whether you're going to charge them, or let them come to you, or whatever. These tactics all turn on how your available weapons match up. Often you have to make these calculations before you even see the person you're about to encounter.
The brilliance of Halo 3 is that you can get some of this information by listening. I play with headphones sometimes so as to avoid waking up the housemates, and one thing I notice all the time is that every significant aspect of Halo's gameplay has a distinct and differentiable sound. Each weapon, each piece of equipment, each vehicle is instantly recognizable. They even have different dynamics; some are loud and some are relatively quiet. It's really remarkable once you notice it. I remember once, when I had been playing Halo for about three months, I heard the tic-tic-tic of a minigun in the distance. And I thought “Holy crap, I don't just know that there's someone using a turret, I know how far away they are from me now. They're on the opposite side of the map but that one gun is louder than the rest.” You can use sound to get spatial information that your eye's can't give you.
On an encounter-by-encounter basis this information is often tactically invaluable. (This is why the game also visually represents sounds using yellow arrows at the edge of your field of vision.) Like, you'll hear that the guy in the room below you has a shotgun, a deadly close-range weapon. Which means: for god's sake, don't just drop in there. Engage from a distance. Or you'll come out of a base and you'll hear a Warthog joyriding around nearby and slaying your teammates well before it appears on your radar. Which means: do some cowering inside the base until you figure out how to take it down.
I think a lot of people in the critical-blogging line don't particularly like what Halo represents. It's a totem for the kind of game (maybe even the type of gamer) us we'd like to see less of. At the very least we'd like to see fewer games attempting to be what Halo is. Hell, even I hate Halo some of the time. But the basic truth is that good design conquers all, and this is where the game shines.