Horizon Zero Dawn is a very good game that will keep you hooked for some time. It offers a well written and more or less original story, though it's gameplay feels a mashup of elements from games like the Witcher, Tomb Raider, Far Cry, Last of Us, Uncharted and even a little bit of Skyrim. An experienced player will notice that most of the gameplay feels a lot like Tomb Raider, the story feels like a combination of Witcher and Last of Us, the navigation and map feel a lot like Skyrim, the presentation and menus are similar to Far Cry etc...
It is cool and satisfying to hunt and kill some monsters for a while, but it easily gets old. Good thing is that the game throws the new gameplay elements in a good way so that there is something new at least for half of the game.
All these do not mean that Horizon is a bad game. It is a very good game, but it's originality lies more with the story and less with the gameplay. Should you forget everything else that you have played, it is a 9 for sure. But for someone who has played all of the above titles everything feels a little "been there, done that"... I don't know if it is fair to judge a game this way, but that's how I see things...