it's somehow one of the only action / TPS games with actual encounter design
most if not all other action games just throw enemies into a room and call it a day, whereas RE4 mixes it up constantly. things like the water room, the earlier stuff with the beartraps and tripwires, the maze, the ski lift thing, the mountain pass, it creates a challenge not just by throwing enemies at you but by making the placement of those enemies and the environment a challenge on top of it
that's not even mentioning the actual mechanical depth, which has also seemingly been unsurpassed in the TPS space