What Selverien said. For the record I have leveled all the way to 58 (before going to Outland) without ever setting foot in WPL, EPL, Blasted Lands and Burning Steppes. And I mean that literally, I have simply never been there. I did Un'Goro until about 53-54, then Felwood, then Silithus (which, contrary to most, I have enjoyed a lot, especially the pod chain quest that yields 19k xp just to die in a tower). I have done a couple of quests in Winterspring and I also "backfilled" in Azshara and Searing Gorge because I had overlooked these areas before (and they were, as one might expect, very easy to do at lvl 53-55, while still giving good xp).