Comment by ScarletEmerald

Graphic design played a large role, I'd guess. Warcraft imagery had this unique mix of cartoon, fantasy, and realistic in a way that most of its successors (and even later versions of WoW itself couldn't match. Plus, the open world aspect of it was the first time many players had ever encountered something like it. There were loading screens only for dungeons and continents. You could walk, run, or ride in Azeroth for hours without anything from outside intruding on the experience.