Moss in particular is one of those things that add real character and familiarity to a building. Ours generally "grows" in the right place, too which, as Bryophytaphiles will know, is in very moist, shady areas and often on the north sides of tree trunks (in Northern latitudes at least).

But our grass is prettier than our moss. It animates, for one thing. Wind blows it, and it's affected by other things. Like explosions. In Halo 2, if you shoot (our admittedly very sparse) decorators with a rocket, they just sit there, glibly ignoring the explosion. Our new decorators are actually destructible. And they cast shadows, and can be lit correctly, like everything else in the game. This isn't stuff we're inventing - lots of folks use similar techniques, but when you see how they're implemented, it really starts to color and enrich the game. Poor old multiplayer has looked sterile and lifeless for too long - and this (among other things) are going to add vast levels of graphical loveliness.

And that's before the nearly-ready-for-primetime water and atmospherics are implemented. It used to be that adding fog to a 3D game actually saved you processing power since it meant that less geometry was being drawn. The way that fog and atmospheric haze are added to Halo 3 actually cost processor cycles, since the geometry is still visible, but realistically obscured by varying levels of particulates.