Artificial intelligence is one of those interesting aspects of a video game that you don't tend to notice unless it isn't done well. Good A.I. is one of those things that blends into the experience, feeling natural enough that you never question it. The enemy soldier flanking you, the grenade just lobbed your way, you accept them because they are meant to simulate how such opponents would act in reality. Good A.I. copes with your good decisions and exploits your mistakes.