First thing has got to be that trees move in the wind... that blew me away, first time out in the woods near White Orchard at dusk and the little trees are swaying with the wind, it was just so amazing to me, I still tell everyone about it when I talk about the game.
I also love Geralt's facial reactions of mirth, he never laughs but you can see his real opinion about others in his eyes and little facial ticks.
I also love the way the roads are in the game, too often I see these medieval fantasy games with paved cobble roads every which way and that... here I was astonished to see that even common roads that cross streams might not have a bridge, the puddles in the road, the grass that grows up between where the cart wheels commonly tread, I mean if you think about it, a stone or wood bridge in some shady lil podunk medieval village would have long since been scavenged for building materials by the local populace.