What I can say is that the stories, characters and choices etc in the witcher games were much more interesting and morally ambiguous than other modern rpg's. You don't have your standard "oh I'm gonna be a bad guy so I only choose asshole dialouge options" or the same for good guys. The world in the Witcher series isn't that kind of black and white
The series also handles mature stuff infinitely better than any other games I've come across. Bioware does this horribly for instance, they almost make it into a minigame that lasts the entire damn game if you want to have sex with someone. In the witcher series that shit is just there, it doesn't devolve into this "gotta unlock the sex scene" . Geralt's an adult dude, sometimes he has sex.
As for Witcher 3, it looks like the world will be very big, and still full of places, people, secrets and quests. Honestly watching the gameplay footage out for the game would probably sell you more easily than I can by writing about it.
But basically the Witcher series excels at delivering interesting stories, characters and player decisions. And Witcher 3 looks like it will deliver on those fronts, while also offering a huge and beautiful world full of stuff for you to do.