One of the reasons I'm so addicted to playing Warriors (and I'm not alone, it seems, as this post from Mike brought to my attention) is that I got to have that feeling multiple times, once with each spec/character I leveled to 70. It moved me from a diffident tank to one who would willingly try and tank any mob in the game, knowing that my skills were up to the task. Similarly, I've had that sensation of 'Ah, that is how a shaman kills three mobs aggroing at once' as well as 'Do not die on me... I love you, Nature's Swiftness' by leveling my two shamans, and even my tiny ret/prot pally has given me that sense of wonder and accomplishment recently by being the hero on a Archaedas kill.
Now is when we turn to you, gentle (and not so gentle) readers: have you had a breakthrough while playing that revealed a whole new facet of your character? Done what you would have believed impossible, saved the day, or just learned something new about your class? Felt like you took a step towards mastering the playstyle or even just figured out that an ability actually did something cool? The comments await you.