A friend of mine and I recently discussed ways to obtain gear at level 70 in World of Warcraft. We talked about potential quest gear, gear from rep grinding, PvP gear, and gear from running various instances like Karazhan. The conversation then shifted to discussions about the Burning Crusade expansion and its impact on gear. Since I never raided prior to Burning Crusade I can only imagine the frustration experienced by a pre-Burning Crusade raider once the expansion went live. The time investment required to attain great raid gear suddenly seems pointless when the gear becomes obsolete shortly after beginning quests in the Outlands. For that reason alone I can understand why some people decided to quit WoW once BC arrived. I could somewhat relate as I watched my hard earned PvP gear from Alterac Valley quickly become useless in the Outlands.
Of course, the same thing will inevitably happen once the next WoW expansion pack, Wrath of the Lich King, arrives in 2008. I'm sure that within a level or two, most of the epic gear we are all currently working hard to obtain will become obsolete. But, after years of playing MMOGs, I've learned that the only thing constant in an MMOG is change.