There's no one culprit for my ending up ahead of the curve. I took the time to do a fly-by exploration tour of all the new zones on my flying mount, with exp awards from 2.5K to nearly 7K per discovery. I've tried a bit of archeology, which also coughs up exp. I haven't even tried PVP (awards exp these days, though I don't know how much) or 5-man content yet.
I'm also burning through content at accelerated new expansion rush rates, having carefully stockpiled (real world) food to eat this week so I could get home, walk the dog, and sign in without the need for pesky cooking chores. This ironically means that I'm running out of rested exp, which significant slows down the rate of leveling - with full exp, or actual gathering professions (which also award exp), I might be even further along.
One of WoW's strengths is that Blizzard actually has the production values to do all of the major forms of gameplay, including solo, group, raid, PVP, crafting. WoW may or may not be the best at any one of these, but they do an above average job with all of them, which can be a major selling point when trying to convince friends to stick around instead of moving on to a game that will only support some of your playstyles.
In that context, it's a little bit strange that the exp curve breaks if you do anything else other than solo quests without rested exp, and that players can't skip ahead in the storyline within a zone if they do gain some exp somewhere else along the way. The stated intent was for players to do either Hyjal or Vashj'ir, but I've never heard that you're supposed to skip all of Deepholm too.
At the end of the day, I suppose it literally pays off to do the other stuff first - quest reward exp is the only type of exp that converts into gold when you reach the level cap. Then again, if it's possible and even likely for players to hit the new level cap solo using just half of the content, Blizzard probably could have gone with a steeper exp curve.