I've just played Operation Flashpoint Cold War Crisis and comparing it to ArmA II, it just feels like Operation Flashpoint had more "soul" in the game. Maybe it's just the old graphics and the cheesy voice acting for Operation Flashpoint, but in ArmA II, everything feels so sterile, souless, and simulatorish. In summary:
Operation Flashpoint: A video game that focused on realistic combat with infantry and vehicles. (Swat 4, Rainbow Six 3 Raven Shield)
ArmA II: A simulator that was ported to civilian use with realistic combat with infantry and vehicles. (DCS : Blackshark, IL-2 Sturmovik)
The first one was meant for having realism in a video game, while the latter was for how many realistic features they could push out the door.