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?
A pure rush of adrenaline shot through me. A thrill from the thought of facing certain destruction. And diving in anyway. My group methodically worked our way down the pyramid steps, mowing down wave after wave of trolls. The NPCs aided us valiantly. I was throwing so many heals with my druid I thought for sure I was going to run out of mana and we were all going to die. Horribly.
But when the dust settled we were alive. All of us. It was a miracle. And that's when I got the second major "wow!" moment of my Warcraft career. And then I got a little surprise I won't spoil for those of you who haven't run this instance yet.
So what was your first "wow!" moment in WoW?