A lot of ink has been spilled over the latest entry in the Legend of Zelda series, almost all of it highly complimentary. And the praise is well deserved. It is an amazing game. So I won't try to compete with what has already been said.
But I will point out a few things that make it such a treat, from a player's perspective. At least, this player's perspective....
Besides the graphics that are, as might be expected, a serious step up from previous titles, the first thing you notice is that this game skips the lengthy prologue so many Nintendo titles use. In five minutes or less you are out playing the game. And in that five minutes you learn enough to make yourself comfortable with the controls, dressed (yes, dressed), and ready to go adventuring. This fast startup was not only a surprise, but an unalloyed pleasure. I have begun to dread the opening of games because I rarely have the 30-60 minutes needed just to get through the openings! To be able to turn it on, get right into the mood and get started was a sheer joy.
Second, there are squirrels! And lots of other things. but squirrels! I have no idea whether the squirrels (and other wildlife) play any serious role in the game. In fact, it doesn't matter. Just watching them hop around and then scatter at your approach is delightful — one of the innumerable ways the game draws you in unrelated to the challenges and mechanics of game play. I could spend days just wandering around without taking on any challenges just to revel in that sense of wonder and newness the game aims for.
Which brings me to my last point. None of this is new. The graphics are tremendous. But other games have achieved this level of world-building before. Superfluous animal life appears in other games (the birds and lizards in Shadow of the Colossus come to mind). Other games have cooking (e.g. Monster Hunter). Ditto climbing and limited stamina (again Shadow of the Colossus and Monster Hunter). The same is true for large open worlds (name your favorite: Red Dead Redemption, etc.). So it is not unique play mechanics or game features that make Breath of the Wild stand out. It is the whole; the sum of the parts and how they are used in just the right amounts to make a wholly engrossing, delightful, internally consistent, and surprising world that makes the game the major achievement it is.