What makes it a great game is not the gameplay, obviously, but the level designs (think the cityscapes from the most recent "I Am Legend" film), the atmosphere (even when you're safe, you can't help but be scared that something is going to burst around the corner), and the story.
The story in The Last of Us is brilliant, with a heavy focus on how the characters deal with the world - two main POVs. The first is a man who can remember the world before the infection, and the second is a fourteen year-old girl who only knows this world.
I totally agree with all the reviews that said TLoU is a genre- and generation-defining game. With the possible exception of the recent The Walking Dead games, I've never seen or heard of a game that deals with an infection-based apocalypse quite so well, with a story this deep. And although the gameplay doesn't bring anything new to the table, it doesn't detract from the rest of the game.