You can't just discuss WoW environments without bringing up sound design and weather effects. Graphics are merely one aspect of why WoW felt like a world.
Ever been to the Shimmering Flats in old WoW? How about at night?[1] It's an entirely different feel (and then you know why they call it shimmering). It's an amazing experience to go from day to night in WoW. Your friends might drop off for the day. You're just alone in this vast quiet world. And it rains. Or you enter a tavern and it feels warm thanks to the welcoming music. How about Duskwood during Halloween? This one game can evoke so many different feelings depending on which zone you're in, what time of day it is, and what weather effects are going on.