The first person shooter has become very regimented in the fact that all the mechanics work around cover-point.
Spark really wanted to make a game that was about movement. The fact that the player has to move and has to move in open spaces is a fresh style of gameplay in first person shooters. It's pretty amazing to me that people don't do more of this sort of gameplay.
The mythical creatures in the game are coming after the player very aggressively. They're walking on the walls, the ceilings, they're making themselves hard targets while they come after the player. That forces the player to move.