I choose the annoying people that crouch or lay down or jump when shot at. It kind of ruins it for me.
However, the unrealistics go in both directions. For example, in reality, people don't run around like goddamn frogs on steroids, they have great difficulty carrying, much less aiming, LMGs, they can generally be trusted to die when hit once or twice with a bullets of any discription, they cannot reload any weapon by lowering it below their line of sight for a second, and finally, wounds do not heal completely with the application of minute amounts of TIME.