In the last expansion, all we did was walk through a portal and find a new world -- the old one was left pretty much unchanged overall. But in this coming expansion, Blizzard has made it pretty clear that almost nothing is off limits -- Northrend is getting added to Azeroth, and we're not traveling across space to get there. And considering that Arthas' story echoes throughout almost all of the old world (and that the whole thing is named after him), things are definitely going to change once Northrend appears on the world map.
So we've put together a gallery of just what will be different about the old world after the Wrath of the Lich King comes upon us. To be fair, some are set in stone (we know for sure that Naxxaramas is exiting stage left for a bigger part later), and some are little more than speculation (Uther played a huge part in Arthas' life, so his tomb seems like a good stage for an event at some point), but odds are that by the time we've all reached level 80, all of these places in Old Azeroth will look very different.
Gallery: Changes coming to Old Azeroth