There's a discussion going down on the forums about what many consider to be the most exciting part of MMO games, including World of Warcraft-- world events. As many drawbacks as there were to the AQ events, no one can argue that they didn't affect everyone in the World-- everyone was able to step up and help the war effort, and even though everyone didn't fit in Silithus, everyone came across some of the invaders that spread across the world. While the Naxx event was that way as well, we haven't really seen anything else on that scale even in this, the largest MMO ever. And let's not forget that since Burning Crusade was released, we're dealing with a lot more people-- there are huge amounts of the population who have never seen a world event yet.
Which makes it all the more likely that we will see a big one for Wrath of the Lich King. Bornakk says that while Blizzard realizes that a lot needs to go into creating a big world event, they've also learned a lot from Brewfest and the Hallow's End events about how to both tune the scale and get players involved. But Blizzard still wants to do a big one, and from what we heard at BlizzCon, they're going to come up with a reason for Alliance and Horde to head up to Northrend after Arthas.
My guess? Arthas is going to become a threat again-- he'll invade Undercity, or bomb Darnassus, or somehow attack our world. And for the first time that I can think of since the game has started, we're going to actually lose something-- an NPC, a city (at the very least, Dalaran) or something else. For all of the fighting going on in the world of Warcraft, it hasn't yet changed that much. And I think that with the next expansion, Blizzard is looking to have Arthas shake things up a bit.