The Cataclysm event in WOW

The Cataclysm event only affected half the capital cities of each faction; every other city, quest hub, etc, was unmolested. Players could play in peace if they were in Outlands or Nortrend at the time, if they were using the WOW gold quest hubs outside the main cities, or if they switched over to the unaffected capitals. The Zombie Invasion, on the other hand, affected all cities, including Shattrath (where a bug between Sanctuary and the Zombie status meant that zombies could kill NPCs at will but players couldn't fight the zombies); the Zombie Invasion even spread to the more used quest hubs outside main cities, where players would go kill all quest givers just to mess with the players that didn't want to participate in the event.

 

The Cataclysm event had guaranteed windows of opportunity for players to interact with the NPCs of besieged cities between the attack waves. The Zombie invasion had no such guarantee; depending on the server, specific NPCs stayed down for days at a time. And zombie players loved to target the NPCs that would cause the largest possible disruption for players that didn't want to participate. Even the Cataclysm event might have been too intrusive, as Blizzard made the MoP launch event even less intrusive; it became effectively an opt-in event. From all I've seen the Zombie Invasion was far more disruptive than anything before or since, at least in populated servers. Seems like it wasn't that bad on low population servers, but that is no excuse really, as expecting players to change servers due to the event would be outrageous.

 

The sad thing is that, had the players participating in the event merely respected the players that didn't want to take part, it could have been perfectly acceptable for everyone, not just for those that enjoyed it; and, if that had happened, Blizzard could have used the Zombie Invasion as the blueprint for other launch events, and perhaps even for one or two seasonal events. Instead, we got enough WOW gold players intent on ruining the game for everyone that wasn't enjoying the event to provoke a huge outcry from part of the player base, almost assuring that other similar events won't happen in the future.

source: http://www.iurpg.com/news/The-Cataclysm-event-in-WOW.html