Well, it's hard to tell without playing but i know this from playing Witcher 2: it looks more dynamic and polished. So it should definitely be a step up. Witcher's 2 combat system reminded me a bit of Dark Souls, it's harder than your average RPG. It's not just hack and slash, you have to roll, time it and then attack.
Also if you played The Witcher 2 you know how good the story is and their dialogue system is the best out there, at least in the genre of Fantasy RPGs. It's as good as Tell tale games and that should tell you something.
So you add a great story with a great dialogue system to a great combat system... To visually what looks like the best looking RPG ever made - cause let's be honest here, RPGs never were that great in the graphics department, this one looks as good as any AAA game - you know you're getting something special.
And last but not least, it's CD Projekt. You know these guys make the extra effort to give their customers what they want. They're one of the few developers you can trust today.