Rather than quests giving experience, zone completion would.  With this model, a zone designated as level 20-25 would take the player from level 20 to level 25, no more or less.  Quests are already grouped into hubs, so give each hub a level, or a level for two hubs, however they need to be divided up to fit the zone leveling.

Obviously mob kills, dungeon quests, and heirlooms are all unaddressed problems.  I'm ignoring them just to focus on the zone leveling idea.