(Howdy, I'm new here.)
One feature of OFP that just filled me with glee every time I experienced it:
Speed-of-sound simulation. If someone was firing a supersonic sniper round at you, you'd hear it hit the ground beside you before you heard the report of it being fired. Similarly, if you fired a tank round at a distant target, you'd see the explosion a few moments before you'd hear it.
Incidentally, does anyone know if this feature is in OFP:E?
The recent videos of Armed Assault seem to imply they've taken this feature out, and that makes me sad, though it could just be the way the trailers were compiled.