The experience and levelling is very different to many games. It's not 'grindy' like most. You get tons of XP for quests, rather than mobs.
The writing and voice acting in TW3 is completely unparalleled as far as I'm concerned. The amount of content and depth to the characters and quests is just insane. Walking through a bustling city in the rain and heading into a busy tavern is completely immersive, where as in Skyrim (which I also loved) the cities never felt anything more than a few houses with 4 or 5 NPCs wandering around.
Gwent was great too! Did you get into the Beta of the standalone game? It's good fun.
Possibly the best game I've ever played. Defo the most immersive, I know that graphics and music don't really add anything to the gameplay but here they added to the enjoyment of the game. I often found myself humming the tunes when I wasn't playing the game.
The closest I got to playing a game like Witcher previously was Fable but this is on a completely different level. But you can do as much or as little as you want. I never bothered with Gwent and only used a few of the potions but I was well into the swords and augmentations and that.
One of the best things about the game was going exploring in the wilds and coming across some monster way too powerful for me, running away and then coming back when I've levelled up.
Then of course there's the patches which improved small aspects of the game and the dlc's which added much more.