Something very special about WoW has been its few, but game community engaging world events. As described by Wowwiki, these world events have invited every player to take part in something that eventually changed the game world. Most of us old veteran players have very fond memories of these events, and I myself have participated in them all, five to date, albeit in quite varying degrees.
The biggest world event was probably also the first one - the Ahn'Qiraj War Effort. At the time (around the end of 2005), I had just become my first own account and wasn't really paying attention to any of the big stuff going on. I did understand something was going on, it was hard to miss, but I hardly took any part in the epic quest chains. I don't even think I turned any cloth and such in, shame on me. I don't think I grasped the magnitude of this, or the time spent by some, until later. At the time I didn't even have a max leveled char however, so maybe I am somewhat excused. I did kill elite bugs in Barrens though, come to think of it. Wow, that feels like yesterday.
The war effort was really a huge effort. Enormous amounts of items had to be gathered, which no one person could do of course. Tens of thousands of bars, cloth and alot of other stuff had to be turned in to a special NPC. The point was that each faction, and the whole server, had to unite in the effort to make it all the way. I know hard core guilds even agreed on who would get the honor of completing the quest (only one person on each server could!), and combine the forces, just to get the thing done and the new übercool raid instance opened. My brother was part of a raid guild at the time, but not the biggest one on the server (if I recall correctly). He was there for the opening of the gates, and so were thousands of other people (but not me, since we only had one computer and well, it was used by my brother :P). The servers could hardly handle it, and our poor old computer showed seconds per frame rather than frames per second. It was extremely cool. The whole event was the very idea of epic. I am sure that anyone who was there will remember it for the rest of their lives.
The second world event was with the Scourge Invasion, and was a prelude to the opening of yet another raid instance - Naxxramas. I remember it quite well. In fact I remember it so well I thought it had been the prelude to Wotlk, until I read about it on wowwiki. "It was for Naxxramas? But it feels like last week?". Yes time flies by and all that. Me and Love went to Winterspring (and some other places) I recall and killed bunches of undeads. If you killed many enough you could get special elites to spawn, which dropped epic items! From this event you also got a special item, Haunted Memento, which gives you a ghost that follows you around if you keep it in your bags. Sort of like a pet, but always around a bit behind you. This item isn't attainable any longer and therefore quite valuable. I've got one on my protadin.
The third world event was the opening of the Dark Portal, with the coming of the Burning Crusade expansion. It was quite similar to the Scourge Invasion with packs of mobs (demons and the like) popping up at different areas of Azeroth. I remember standing in front of the Dark Portal with hundreds of other players, trying to get kills for a special quest. You had to kill Invading Felguards that spawned by the gates, and it was extremely hard for me to tag a kill as a priest. At the time we didn't have skills like Shadow Word: Death, and you didn't tag a mob before you did damage to it (unlike now where any action against it will tag it). Love had no problems with getting kills of course, being able to moonfire anything he saw. I eventually managed to get my kills however and was rewarded the Tabard of the Protector. Yet another item which is no longer attainable.
The fourth world event was for the Sunwell Plateau raid. A whole new island was introduced and you had to do continuous questing in the area to help the elves take it back from Kil'Jaedens minions. By questing a new area of the island was claimed by the horde/alliance, and new quests where made available. Since I hate daily quests, I rarely did any on Sun Reach (the island). But I know plenty of people who went there everyday, doing those quests. You got a satchel with rewards from some quests if I recall correctly, which could contain emblems. And emblems were already then quite sought after. One thing that was really annoying about this event was that one of the questing areas was just around Lord Kazzak (if you were unlucky you accidentally pulled him and died quite fast). He was residing (and still is afaik) in the northern parts of Hellfire Peninsula. He is an outdoor raid boss (they had them back in the days, but removed them for wotlk for some reason), so you're not suppoesd to engage him. Everytime you do he starts yelling alot of stuff, like most raid bosses do, all the time (thinking of Putricide). But since so many people were questing around him, people aggroed him constantly and he was yelling 24/7.
I've got "ALL MORTALS WILL PERISH!" and "THE LEGION WILL CONQUER ALL!" and then when he had killed the poor sucker two seconds later "CONTEMPTIBLE WRETCH!" burned into my memory from hearing his spam yelling for too long while questing in Hellfire...
The most recent world event was as long ago as October 2008. What also happened around there? You guessed it (maybe), the release of wotlk. The world event was the Zombie Infestation, a funny thing where people were infested with the zombie plague and turned into zombies (with special skills!). It had varying phases. At first you could see strange looking crates standing around in Booty Bay, if you touched them you got infected. Argent Healers started turning up in every capital city. Eventually the crates found their way to the capital cities, and so did infected roaches. When killed they infected you. At first the disease can be cured by spells like "Cure Disease" and the like, but over time it becomes more and more resistant to these type of spells. People turn into zombies faster and faster after being infected. Infected rats turn up as well, which eventually even attack people, consequently infecting them. The zombies also become more and more powerful. Something close to chaos ensues as everyone is turned into Zombies all the time. It was quite entertaining.
Finally, a cure for the plague is found and the plague slowly disappears.
Well, there we have the five world events that have been (there also was the unplanned one). Now 2,5 years have gone by without any world events. Some could of course count the Argent Tournament Grounds as a world event, sure. It is somewhat similiar to that of Sun Reach, but not really of the same magnitude. Blizzard must be loading it for the big bomba. Literally. The biggest change ever is going to take place in their game, that of the Cataclysm. I strongly doubt they'd simply have the entire world change completely over night. There just has to be some sort of enormous world event preceding it. Smaller happenings have gained this honor. Some say that the new raid instance Ruby Sanctum is part of this. We will get to learn more about Deathwing and why (and perhaps how) he'll turn the entire world upside down. But I am really hoping for something huge. Something enticing. Something that would be like all the other world events combined. Or at least close to. I could go without all the collecting of items you had to do for the AQ event ;) It has to come soon, if Cataclysm is released within this year. And I can't really imagine what it could be. Can you?