I think one thing that OFP did amazingly well, that ARMA1 and ARMA2 didn't do well unfortunately, was creating a realistic speed for units to fall to the ground when shot.
When you shot an enemy in OFP they hit the ground quickly and realistically. There were almost no slow motion type deaths and the movements were fluid. In ARMA1/2 the unit deaths are way too hollywood style. When you shoot a unit they should drop to the floor hard, not slowly fall to the ground. I'll have a look around for some videos to use as an example.