I posted the following in a thread on the Steam forums, with the thread title "So.. Game of the Year??". I liked the way the post turned out, so I wanted to put it here, too:
It advances the art of storytelling in games.
Videogames are not a necessarily narrative medium (e.g., Civilization is not about telling stories). But storytelling is one of the things you can do with them, and like any other medium, games have their own unique strengths and weaknesses.
One of the strengths is identification and sympathy with the protagonist. In books or films, for example, you sometimes have to work to make the audience care about the main character. In videogames, because you are controlling the actions of the character, that identification comes practically for free, and it tends to be quite a bit stronger. The Walking Dead uses that very effectively: It's not just that Lee cares about Clem and wants to protect her, we the audience care and want to protect her.
Thematically, the game is about choices. I've seen people say that the main element of "game play" was the quick time events, which I suspect is missing the point. The real primary game mechanic is making decisions, under time pressure. This is underlined in the very first episode, when there is an entire conversation about how in the moment, it doesn't even feel like a choice. Everything that happens is about choices, and their consequences, and everything from the story to the dialogue to the game play supports that theme.
Even to complain that in the end, the choices don't change the outcome, or that you might as well watch a video replay, seems like missing the point. Whether you have real agency or just the illusion of agency ultimately isn't important to the story they're telling, or the feel they're after. The fundamental thing that makes videogames unique among storytelling media is the interactivity. In an aesthetic sense, this game isn't about seeing "what happens next", it's about the visceral experience of making choices in the moment. The plot is just the framework that experience hangs around. It doesn't need multiple endings, because it's not about the ending, it's about how it felt along the way.
Even knowing whether or not there are multiple endings requires knowledge from outside the game - either replaying and making different choices, or reading spoilers. I made a very conscious, deliberate decision early on to not do those things. I gave myself a single, blank-slate playthrough, no spoilers, no do-overs, and it was the most powerful gaming experience I've had since, oh, let's say, Grim Fandango. Game of the year? I say yes.