One of the most significant aspects of the redesigning of the old world is the level adjustment of almost every zone.  Starting zones will remain 1-10 (1-12 actually), and secondary zones 10-20, but from then on everything is fair game for changes. The designers' goal is for every zone other than the first two to keep players for about 5 levels, but that's not the biggie here. After a certain zone has depleted its quests, it should lead the players to another zone of appropriate level - right next to the one that you just completed. No more traveling from one continent to the other for this 1-2 levels that are preventing you from continuing your questing! If you start in Northern Kalimdor, you should be able to level to at least 55, possibly 60, before having to move to another part of the world.

The maps below indicate the level range of each zone the developers have announced ready. Some zones are missing their levels, but that's because nobody knows - with certainty - what those levels will be. Also, Deepholm has been tossed on the Eastern Kingdoms map - it doesn't belong there, we all know it, but it was the only zone that didn't fit anywhere, so EK adopted it.