The best Mil-Sim/FPS game you can get your hands on, and better yet, its free
Some aspects that make it so realistic is the slowed pace that the player moves at, the ability to crouch and prone to improve your accuracy, the use of actual gun sights for aiming rather than a cheesy zoom in, weapons can actually jam, different firing modes (semi, burst, auto), and the fact that at most it takes five shots to kill another player.
Graphics for this game could be better, i run it at full and its still looks like halo 1 for PC but hey, the game is free. However, one thing this game has over others is its near 100% accurate weapon models since so far i have one little discrepency with the M16 model and its only there to save polygons. Reload animations are true to life and the character models are just as good, though they can be improved on by today's standards
Sound is dead on. You hear every click when reloading a weapon, even the reloading of other players. The gunshots are perfect and are scaled correctly to the rest of the soundtrack (significantly louder than anything else you will hear in the game, aside from a grenade at your feet) and its real easy to distinguish which gun is being fired. Ambient noises like distant gunshots and animal ambience give a real feel to the environment and makes the game that much more immersive. The sound of bullets whizzing pass you makes the game feel more real and the different impact sounds from bullets hitting ground to bullets hitting wood is just as accurate as any other sound in the game.
Controls are pretty much like any other fps, wasd to move, mouse to aim and other standards. however, one thing that might turn away new players is how many controls there are. From calling out positions, whether to go prone or crouch, switching grenades, putting on a silencer, fixing a jam, and performing actions, this game might be a little daunting to some. As for how the game itself handles, its pretty solid but it could be a little better, if you are sprinting and want to go straight into prone, your character might idiotically just stop, go down, then stand up again, making the game feel a bit clumsy sometimes but it wont let you down when you need the controls to work.
This game is the most realistic thing you'll find out there but i will say that its not for everyone. Halo players that are used to playing Rambo and taking a magazine and a half of bullets might find this game to be frustrating but a game that requires teamwork and tactics/strategy is much more refreshing and rewarding than mindless 'running and gunning' and 'shoot/kill/repeat until killed'