Now, I'm sure you've all reminisced your way through a handful or two of the "which was your favorite expansion?" conversation. Our geologic time scale will help us slice straight through to more specific ages. We all know how radically different the beginning and end of an expansion can be. Some players relish conquering the learning curve at the beginning of a new expansion and pushing hard through new raid content; others enjoy the long glide once quirks have been unknotted and encounters are freely available via LFR or farm mode.
Me, I have a soft spot in my heart for what I think of as the Early Heroic Age of The Burning Crusade. I enjoyed the long process of toiling through the various reps and attunements before I could even step inside the intimidating 5-man heroic dungeons -- only to have my corpse smeared on the cave floor by a whole host of heroic 5-man bosses after picking our way through trash pulls that insisted on terrorizing us with more mobs than we had crowd control to cover. Those early days of mastering brutal pulls and creatures (Underbog Lords, I salute you) while wearing mere quest gear were some of the best times with good hunting buddies that I've ever enjoyed in World of Warcraft.
In your heart, you undoubtedly cherish what you consider your own Golden Age of WoW. When was that -- the beginning or end of an expansion? A particular patch? Or, like me, was it a specific content curve tucked within a stretch of time that encapsulates everything the game can be for you?