McGraw of the Drenden server asked an interesting question on the official forums: What was your first "wow!" experience in WoW? Replies vary from discovering the autorun button to flying on a griffon for the first time. Many people spoke fondly of their first ragtag outdoor PvP raid. Even a CM chimed in about watching the flyover video when entering the game and realizing that those were actually players running about.
For me, it was much later in the game. I remember adventuring in Zul'Farrak for the first time in my 40s. My group and I fought our way to the top of a pyramid to save a group of NPCs. When we defeated their troll captors and freed the NPCs, we turned to escort them out of the zone. That's when I saw the first group of trolls spawn. Then another. Then 5 more. Then 10 more groups after that. It was like looking down into a canyon of swarming death and I thought to myself: how in Elune's name are we gonna survive this?