As with most modern MMOs, the endgame begins at max level. In order to help players level faster, Blizzard added a heirlooms mechanic in the second expansion (Wrath of the Lich King) to allow low levels to have above-curve gear while leveling, with a 30% EXP boost. The third expansion (Cataclysm) revamped Azeroth, making new questing much easier and straightforward, and therefore less time is spent in the old world. The fifth expansion adds Dungeon Finder, which is a faster way to level than normal questing. So there is not much incentive to explore until you hit 90 and can go to Draenor. And then once you hit Draenor, you have access to Garrisons, which is a FarmVille-like mechanic which has the easiest financial ROI in the game, making even less of a reason to just explore period. I did not renew my subscription.
Guild Wars 2 had a great solution to this; all zones have content, which the player downscales to. Unfortunately, Tyria (the world of Guild Wars 2) is not as robust as Azeroth.