Problem: Leveling was fun the first time around, a bit less fun the second time around on your first alt, a tad dull on your third, then boring, then please kill me, etc etc.
Blizzard's solution: Heirlooms in every slot, reduce XP required to ding by 90%, make mobs die from being given a stern look, make instances 10 minuter zerg fests. Basically make it as quick and brain-dead as possible.
Better solution: Make the world a living and exciting place. Add events. Make the world constantly changing. Make it change from what players do to it. I believe WoW has the best world and lore of all MMO's out there at their disposal but it's going COMPLETELY to waste. If I could change one thing that would improve WoW it would be this. Have everyone out in Azeroth experiencing and changing the world, meeting new friends and making new enemies in world PvP. Instead of sitting like a bunch of brain-dead chickens in capitals trying to pick gear off a conveyor belt in order to increase their ilevel.