Quest design wise, it is the best RPG I have ever played. It is also the best narrative I've ever experienced in an open world game ... which isn't to say it's perfect but it is overall very good. The dialogue is also about as good as it gets (although I wish there was more ... which in a video game is astonishing). The characters are obviously strong, though they they change less than I would personally prefer throughout the story. The choice and consequence system is better than most games I have played, though not having varying end states does but a blemish on it. Overall narrative experience I would agree it sets a new bar for mid-evil setting RPG video games.
The visuals are better than any other open world-ish RPG I have played (granted I play on PS4 so my opinion in that particular matter is not hugely important). The music is top notch (but lots of games have great atmospheric music). So atmosphere wise it also sets a pretty high bar for this style game.
The combat and movement controls are solid, but by no means ground breaking. Alchemy is pretty good, not my favorite, but I understand why they made the design choices they did for the sake of the open world game. I liked witcher senses more than I though I would (they are much more enjoyable and challenging when the mini-map is turned off - and before the patch were even difficult for me because have a red-green color deficiency in vision ... making following red tracks or scent trails in the woods or grass very hard). Signs are signs ... nothing too great, although granted I have not played a mage build). The UI interface is just okay for me - definitely not ground breaking. The leveling system is not very rewarding, but that makes some sense when you're supposed to start as the ultimate bad-ass... there's only so much progression to be had before you achieve OP deity status. The game play is definitely more enjoyable on the higher difficulty settings. Generally speaking I would say the game play design is solid but unspectacular.
So ... yeah. Where the game really shines is story, open world, quest design and atmosphere. On top of that it does just about everything at least well IMO ... which is a feat in and of itself. How the individual elements blend together for the overall experience is the true testament to the game's greatness. Even the parts that I dislike (with the exception of the UI interface) I can understand why the choice was made to serve a purpose that enhances another element of the game.
