Game design and narrative genius. The writing alone makes this game worth a dozen playthroughs, but so rare is a game that allows you to "do" almost nothing yet feel as though you've lived through unknowable things. Yet, you laughed until you didn't. In a snap the game will flip itself on its head, and your player behavior is always the driving force, even when it feels like its railroading you down corridors.
And the DLC's perspective on success and failure, the ultimate meaninglessness of all these things we quibble and babble about, is so well-fleshed out and visualized, with an ending so chilling, thoughtful, and bleak it's been burned into the back of my eyes. It's an astonishingly brilliant use of the video game medium, and a bright shining light upon what the core of interactive media has always been: first-hand experience.