It's the visceral satisfaction of firing a weapon and seeing it do damage. Gore is only one part of that, environmental reactions should be up there as well. That's why the particle system in FEAR is still heads and shoulders above most shooters. That's how I see it anyway, Raptor is just bloodthirsty.
Uncharted will never have gore though, and that's fine. Enemies have to react to being hit though. Every bullet. That's crucial, and more than just feedback for the player, it adds some strategy by being able to reliably stagger enemies, especially while hanging.