In my opinion, it's almost non-existent. I'm talking about enemies and their reaction to the player's actions.
The good example is Fallout 3/NV. You can wear a full set of power armour and butcher a street thug with your trusty Ripper in front of his 'colleagues', but they are like 'F*ck it. We're still gonna attack you.' What the cheap crap is that? Was it so difficult to include normal fear in their behaviour? Another example. I'm attacking an NCR outpost, wearing mod-added Kerberos armour with huge damage threshold and holding anti-materiel rifle in my hands. All NCR personnel is swarming around me, shooting. Their service rifles did almost no damage, and I, in turn, blew heads to bits, striking one headshot after another. But they didn't run away till the last of them was dead. Veeery realistic behaviour indeed. Crap like that completely breaks immersion, leaving you with the thought 'That's bullsh*t. Real individuals don't behave like that.'
It's 2015, but we have the same zombie-terminators without any fear instead of normal enemies in games. And CEOs keep telling us about the 'developing of the game industry'. U-huh. Right.