I don't know if "impressive" is the best word to describe The Witcher 3. I mean, you don't get to fly the skies, create or destroy the landscape like in other open / semi-open world games.
What you get is a candid, warm personal journey.
How did they manage to nail both -The game mechanics- and -the story telling elements-? Well, the game benefits greatly from the books and lore in general, something that other games have to struggle with. This could work as a disadvantage as well, but the developers managed to build an innovative, unique and organic experience that never felt forced.
I don't know if The Witcher 3 will pass the test of time as well as Skyrim did, but I can say that Geralt and his friends will stay with me longer that many other AAA's out there.