Heirlooms were a huge contributor, but they don't give an experience bonus anymore, so by that logic, the problem would've gone away after that. Of course then scaling was already in place. I guess we can take scaling away then, because the heirloom problem has been fixed.
In reality, the main problem was the reduction of experience needed and the restructuring of how zones work. Zones didn't always have a clear beginning and an end. You weren't expected to necessarily do all the quests there, and it wasn't as linear. Sure story is important, but there's different kinds of stories. You'd pick and choose quests how it made sense to you. There was no overarching story. More like many smaller ones. And you'd often go to a place like Stranglethorn for 2 levels, then go somewhere else and come back later for another 2 levels, sometimes gathering up dungeon quests and then doing a dungeon. Dungeons have a story, too, you know. You wouldn't outlevel much, and if you did get some green or grey quests, you'd just drop'em. There was no reason to do them if they gave low experience. Because the whole point of doing quests was to gain experience. Not to see some overarching, mediocre, linear story.