My theory on zone changes and how it works with Cata world updates is that as you level through each zone you assume that any and all quests in that zone have been completed one way or another. Example being the bridge in Redridge, when you leave that zone the bridge is completed soon after. Blizzard didn't make the game with the technology to change as each player went through and fixed things. This is why suddenly everything is different, technically every player was in Northrend and were not aware of the changes back home. In other words, every player hasn't been back (story-wise) to Eastern Kingdoms or kalimdor for 4 Years.

4? More like 2ish, 1 year for Outland and 1 for Northrend, though we kind of went through Azeroth to get to Northrend, so really our characters have only been away a year or so. It's just that during that year, Deathwing came forth and burninated the land.