Ballistic crack of supersonic bullets passing by has become more common in games, but one thing I don't hear much are ricochet sounds. When ricocheted bullets pass by they make some really cool sounds. It would be nice if this was added to the audio spectrum in the game.
The default birdcage flashhider on the M16A2/A4 and the M4 does a great job of hiding muzzle flash, even in poor lighting conditions. When the flashhider is removed or when a compensator is added, however, the muzzle flash can even become a huge fireball. There are usually two kinds of compensators/muzzle brakes: 1) those that dampen recoil but send the blast in directions that may be unfriendly and obnoxious to nearby friendly troops 2) those that increase recoil but project the muzzle blast forwards. The latter should rattle the senses of any opponent going barrel to barrel with that compensator.
To promote team cohesion, I think a player should be able to take only 1 AR/M16/M4/Mk18/etc. or AK magazine from a teammate's backpack using F. If the player is out of primary ammo, however, he will not have magazines to give up in his backpack. Riflemen might also serve as Assistant Gunners and be the only class from which a Gunner can get an extra belt or box of ammo. Those with Exotic Weapons are just out of luck and will have to rearm from the dead or at ammo boxes. I think this will reflect the commonality of assault rifles and their magazines in military and paramilitary forces.