Many of us get jaded playing WoW. We get burned out running the same raid for months on end as we wait for new content. We feel there is no reason to do instances anymore because there is nothing more to be gained -- or with the new expansion on the horizon, we think, "Why bother? I'll replace the gear with quest blues and greens leveling to 85." We spend a lot of time QQing about how there is nothing we need to do or want to do, or how we are bored with the current aspect of the game.
However, we have all had our Oh wow, that was awesome! moments -- those moments that bring us back to why we play this game and why we pay 15 bucks a month to keep doing so. I have had my fair share, usually a couple in each of the iterations of the evolving world that is WoW. The first one had to be Ragnaros. At level 60, getting to Ragnaros and watching him just erupt out of the lava pool all gigantic and epic-looking ... He is still one of the most impressive-looking bosses with one of the best deaths in the game; it is kind of a shame you can kill him with just one or two people now.
The Burning Crusade's high point to me was Mount Hyjal. I loved the Caverns of Time. I felt it was a great gaming invention to allow you to experience content from the old RTS Warcraft games in a WoW manner; being a part of it was a great idea. Also, Archimonde was a seriously great fight. Wrath has had a couple of memorable moments for me. Ulduar was a huge high point. Yogg+1 or +0 was such a truly difficult and exciting fight. And finally getting to fight Arthas -- that was a very well-designed fight, even in normal mode without the buff when it was actually difficult.
What moments in World of Warcraft have left you amazed? All the bugs, server downtimes and trolls be damned -- this made up for all of it. What instant made WoW all worth it for you?