The forests around it remain quite possibly my favorite environment in any video game. The small hamlet of Flotsam feels like a real and distinct and unique place. It also introduces you to a world that's steeped in lore and history, that feels real and inhabited. From that game's powerful opening cinematic (perhaps the best in any video game) through its cleverly-directed opening segments, to the arrival at Flotsam, The Witcher 2 is simply breathtaking.
For one thing, the opening Act is nowhere near as powerful as in Assassin of Kings.