In the old days zones were structured as follows: Intro quest(s), Exclamation Point Explosions, then Dungeons. The Intro quests were just to give brief exposition; they could be as brief as a one-quest story bit from Hemet Nesingwary in STV, or as intricate as a couple quest chain. the Exclamation Point Explosion (which has all but disappeared in the Wrath/Cata world) was when your minimap would be flooded with fifty quests and you'd refer to Wowwiki or your leveling guide, or your pal who just leveled the wrong way. Dungeons could be instanced or microdungeons, like Jintha'Alor, and you'd have a smattering of quests that let you go into the dungeon and beat up some trolls or naga or what have you. It was not this organized, but it was what was behind the design.
In BC, they refined their ways to the form above. It worked well; at normal speed of leveling, you would get through every quest and every dungeon and be very prepared for the next zone. Then... well, then came XP nerfs (which I won't write about, because they should be taken into consideration when Cata was designed) and wrath. Oh, I should note - you could intentionally make the game easier by doing more lower level quests, or harder by doing fewer harder zones.
Then came random dungeons. Dungeon quests are now offered in instances (so they are removed from the lore of zones), making people think of them more as separate bits of 30-minute content. XP-ridden content. You can level a good number of times without actually doing quests and not getting bored of dungeons. With a new dungeon ever couple levels, it's quite profitable to at least run them a few times and out-level what the zone thinks you should be.
Face it - Wow had 11.5 million players BEFORE Cata. even if a solid million of players now come with Cata, it's ridiculous to assume that the majority will not have played the game before. That means people leveling will be looking at content with experience; this means people will be smarter. Running dungeons when they're bored, killing a few extra mobs for a bit more xp, making sure your characters are well equipped, not to mention bag space, twinking, and heirlooms. Especially not the heirlooms. People will be smarter, and out-gear and level content faster.
A Bloody Ton of Content
Not a whole lot to say here. There's a bloody ton of content, and not only is there lots, but it's good content. Compelling. And fun. More zones = more XP = more outstripping of higher zones.
Why is this a bad thing? Well, if I want to play the game - and make it a challenge - I have to a) skip fun content or b) not finish any content. Both suck. Or, I could cap my XP. Which is no fun, because I still want to develop my character. Solutions? Maybe a Difficulty slider, or an XP slider, or something? Please? Because your game is fun, Blizz, and I want to play it.