Probably three main things-
(1) It bucks the trend of modern shooters (heavily story-based and scripted) and basically just dumps you repeatedly into large, well-designed arena-style levels with tons of verticality, interesting enemies and hazards. You are in control of your positioning, you have to assess everything, etc. This is a big breath of fresh air, when so many games, like Call of Duty or Battlefield or Uncharted, basically take control away from the player and say 'here’s your cover, hide behind it and kill these shooting-gallery enemies, then follow that guy to your next objective, then repeat." Closely related to this, is the movement/jumping system. You can move and jump way faster than in any other shooter, even the new Call of Duty’s with the fancy jetpacks.
(2) The glory kill/chainsaw dynamic. Do I go in for the glory melee kill to get some health, which may risk exposing me to a big crowd of enemies? Do I switch to my chainsaw to forsake that health to get some much-needed ammo? (Granted, it makes no sense that a melee kill would drop health, while a chainsaw kill would drop ammo, but the game to its credit cares about design more than plausibility). Given the scarcity of health and ammo and the difficulty of this game, these are tough choices. You have to make them constantly – it’s thrilling. Like a dance almost. Also, side note, the weapon upgrading is a lot of fun. I really wanted to put a scope on my heavy assault rifle, until I was tempted with the mini-missiles…decided to circle back to the rifle scope later…
(3) It looks amazing. It also sounds great. I love the soundtrack that kicks in during fights. Also, it doesn’t take itself too seriously.
Maybe that’s more than three things!