What a lot of people will notice once getting into the game is how realistic the sound effects are. Bad Company is using a new sound engine with HDR (High Dynamic Range), some of the best audio technology!
Everything from surrounding materials to the environment to the distance between you and the nearest wall, tree or tank is taken into account with every round fired. Each click of the trigger brings different sounds than the next and with so many vehicles lending to the destruction there's no telling how many different types of sounds are actually included.
Not every shot of your rifle will sound the same based on the environment around you. e.g. If you're in open space there will be a long echo throughout the battlefield of your shot and your enemies will hear the residue of the 'boom' km's away. Compared to within houses and trees your shot will sound a lot more quick and sharper.
Just another aspect of the game that excites me and brings a sense of realism to the battlefield.