Imagine an army of hellish demons hunts you relentlessly. They vary in their capabilities but many are a dangerous threat when they reach your immediate vicinity. Their fiendish claws can eviscerate you with a few swift blows. The game I'm depicting is Painkiller. Painkiller utilizes varying levels of enemy spacing to create an enjoyable Doom-style experience.
The most obvious contrast between Painkiller and other FPS's is that the player is generally running away from enemies that hone on him. Other FPS games do feature enemies that are lethal at close range, but they don't always make a direct beeline for the player.
Headcrabs have to see the player before they will engage and the Headcrabs' attacks are separated by being forced to reorient after every leap. Normal zombies in Half Life 2 are completely oblivious to the player until they see Gordon Freeman. Enemies in Painkiller target and pursue the player from the moment they spawn out of portals.
Having enemies that are a direct threat to the player creates for interesting gameplay situations. The player can never relax behind a solid wall of cover, he has to dart around while firing a multitude of weapons at his aggressors.
To complicate this, the game throws in ranged enemies that will periodically shoot projectiles at the player. The player has to gauge threats, decide which enemies are the most pressing or immediate concerns, and finally how to dispatch them. This frequent split-second dilemma makes for tense moments requiring the player to think fast.
The actual environments that the player resides in can provide for interesting battlegrounds to wage war in. The player can be locked in a dank, cavernous tomb or have the freedom to roam around on multi-tiered levels of a ghoulish catacomb. The environment itself is a tool that can be used to slow down or disorient the enemies when used properly.
The player's path that he takes in an environment can potentially be a strategic boon or a crippling setback. Successful or failed decisions usually result in an instantaneous survey of the enemies' spacing in relation to the player, the surrounding environment and a decision of the next best course of action.
Another game that pursues this theory of spacing is Killing Floor. Enemies hone in on the players from the moment they spawn and as a result the player is usually trying to create more spacing between him and the enemies. The environment also serves as a tool to protect the player (barricading doors and holing up in a Cathedral) or a death trap (dying in a secluded corner). Because the enemies follow the players' paths, the players are creating new experiences when they traverse the environment.
What does this mean in relation to other FPS games? I'm not advocating for enemies to be in melee range and to pursue the player at every turn. I do believe there are more opportunities in FPS games to have more than just a x enemy with a gun and a y enemy with a laser.
If there are varied enemies with different behaviors it may trigger this satisfying decision-making process. The more aggressive an enemy is, the more immediate of a concern he may be to a player. Aside from an enemy's characteristics and damage outputs, this could be used to design challenging levels based on a player's perceived "threat" from external sources.
In conclusion, Painkiller may not be the world's most strategic FPS but it effectively features elements that force the player to think and react based on the space around them. And it has a gun that shoots shurikens and lightning.