In the days leading up to Wrath of the Lich King, plagued crates started showing up in major cities, triggering a widespread zombie infestation that, for a few days, brought World of Warcraft as we knew it essentially to a halt. In the days leading up to Cataclysm, cultists started showing up in the major cities, heralding an elemental invasion that would also impact World of Warcraft, just before the game would change once and for all.
With both events now in the history books, which event leaves you with the better memories?
For better or worse, the zombie infestation will almost surely go down as the more memorable event. It involved unique player mechanics not seen before or since and even created a de facto third faction which members of both Alliance and Horde could temporarily join (not always voluntarily). The elemental invasion? Quests were there, you did them, stuff showed up, you killed it. The settings may have been unique, but in the end it was still "just WoW."
Which brings us to the downsides. During the zombie infestation, WoW stopped being WoW. Which meant: Want to raid? Want to quest? Want to farm? Too bad. All that was pretty much shut down, and we weren't being told when it would be back. It turned out to be just four days, but nobody knew that at the time. With the elemental invasion, when players got tired of it, for the most part they could ignore it. Dalaran and Icecrown Citadel were unaffected, and pretty much everything was out of the way except for the city events -- and for those, you always knew that things would be back to normal in an hour at most.
Neither event was without its flaws, and both were unique in the history of WoW. No doubt the advent of the next expansion will produce another such event. Which event would you like the next one to be more like?