One thing i like of WoW is that when i go to the big cities (Iron Forge, Stormwind, Dalaran,etc). i can see lot of pc running around. People forming groups, trading, chatting. I'm usually more a solo player than a team player and if i have to choose i prefer to quest, farm, grind alone than do it with others but i also like to see people running around in some situations. Since Burning Crusades came many places of the old content became desolated. For example while Stormwind is allways full of people Darnassus is usuallly empty. There many people at all time playing around Goldshire but Desolace is desolated almost all the time.
I think Blizzard kept the auction houses and most of the class trainers at old major cities so player would have to go back there again.And that make sence for me, quite depresing to start playing a new mmo and have the feeling that you are the only one playing it.
Blizzard could add a new continet or dimesion (i'm still waiting for the emerald dream... lol) but that probably would have made the old zonesmore the desolated, specially the areas in eastern kindom and kalimdor, since now with the group finder is much faster to level in instances than to quest in those areas. And at the other most of the people tend to rush to end content game and also tend to skip dificult or boring quest. And to make it worse if the area is already empty is almost imposible to find other players to run those quests.
I like the idea behind the new expansion cause i think it will bring back players to those areas even if they dont look and feel like they used to.