The Direct Communication feature should be higher in volume, reach a further distance, and drop off in volume less sharply. If it were possible to use direct communication to speak loud and clear to people 10m~15m away, which is the typical distance between people when fighting or marching, as opposed to the current ~3m away, then my Arma group and I would probably have no real use for those radio mods like TFR and ACRE.
It's a small thing that would make a big improvement to the game.
//In other words: please boost the volume and reach of the Direct Communication voice-over-network channel so that it is more practical to communicate with your teammates when they are within 10~15m.//
edit-- here's an illustration of what I mean because people like pictures: