I agree that you level too fast through the old zones. There is very little sense of threat or danger. Even only using the gear that you earn from quests (ie no heirlooms etc) it is very hard for your toon to die. I have even tried to pull packs of mobs to try and kill me and the usual effect is that by the time I manage to collect enough mobs to be a danger (about 6 or 7 mobs), the first two mobs that I have tagged have gotten bored and reset.

If the challenge becomes 'how do I keep 7 mobs tagged' rather than 'OMG I've pulled 2 mobs at once, HOW will I survive?' then that's a boredom inducing problem. I've levelled 3 or 4 toons to 30 or 40 in the new zones but have given up on them because it's wayy too easy.

I believe that a solution to make questing more challenging/difficult for experienced players is to raise the barrier to picking up new quests.

Right now you can't pick up a quest that is more than 5 levels higher than you (* I think it's 5). If they made this number 7 then players who wanted a challenge could grind through the more difficult mobs and quests. A player can make their own choices whether it's too hard or not and drop back a zone or two if they are getting stomped on too easily.
People who want to zoom through the levels can still do so, but those who want to stop and smell the roses are missing out.

They have done a great job making the storylines in these old zones epic and compelling, but the gameplay suffers if you are fighting easy grey mobs by the time you get to the end of it- even if you start the zone at the absolute minimum level.

Let us take quests at lower levels and bring back the sense of danger!