It combines...

- Heretic and Hexen Nostalgia. - Pace of the action, art style and weapons designs remind me so of those magical and dark games. You don't need the pinpoint aim of a ranked Battlefield 4 or Counter-Strike player to enjoy this game.
- Short easy to digest play sessions. - This isn't an MMO or Civ game where you need to block out 2 hours of time to enjoy the game. You'll either die quickly, or get through a nice short level in one sitting.
- Randomized levels and weapon pickups. - Each play through is different enough that it doesn't get stale quick.
- Extensive unlocks for those who like investing more time. - The more you play, an even wider variety of weapons and leveling options will appear.
- Challenging. While it's fun. It's still quite challenging. You need to carefully decide which leveling bonuses to take as many have both positive and negative bonuses.
- Warning. It does have perma-death. But re-leveling is very fast and lets you try many different leveling choices/paths.

My top pick for single player action game of 2014. Yup, you heard that right. For me it easily beats out other great AAA titles like Shadow of Mordor. Sure, some other titles have deeper more rewarding stories, or more complex gameplay. But this is simple and fun with just the right amount of nostalgia.