Dying Light's initial release came out at the perfect time. It really capitalized on the cultural obsession with zombies. I sort of understood it, and I enjoyed it too, but I never really found "my" zombie thing until I saw Dying Light.
The gradual increase from weak and incompetent at fighting, slow and awkward at parkour, to chopping zombies in half with one strike, and passing over buildings like water, is so incredibly satisfying. You really do go from an incompetent feeling nobody to feeling like a living legend.
The gameplay and exploration is where the game shines the most. Exploring Harran, gathering supplies, and taking time to learn where things are in the map when you need a re-up makes the world quickly become easy to memorize, and its small size and the movement being really fun make traversal a joy.
The extra difficulty modes and free added content added a lot of replay value for me. Night runs were also a brilliant idea, letting you explore in the dead of night with more zombies shambling around and the dangerous volatiles, which will give you a progressively more hectic chase until you reach a safe zone or the sun rises. At night, your XP is also doubled, so it's an extremely smart way of having a built in difficulty that you can easily opt in and out of. Much like zombies themselves, Dying Light will have its period of dormancy, but it always seems to come back for me.