I remember the first time going into Ironforge, and being immersed completely like nothing else before or since.

Even my first Oculus did not beat that.

I'm not sure what it was. The graphics were far from ground breaking. There were other games with big worlds. But I think the combination of world building, graphics, scale, and quality of execution (nailing the style) just completely shattered the level of anything else that came before - or at least for so many people that played the game.