There are enough leveling zones, now, that they could design half of the zones to be challenging and half to be easy. I honestly think that would make things better, granted it would cut into new content (cataclysm revamp again). Once a player makes it out of the starting zone, they can opt to take the hero's path towards the highest level or to take the normal path towards the highest level. Hero's path would be challenging and possibly designed around players with a full set of heirlooms, which would make the leveling rate in each zone the same as the normal path's zones (by this I mean a normal path would take a player from say 12 to 20 whereas the hero's path zone would take a player from 12 to ~26 with the end quests being level appropriate still).Could make the case that WoW has enough cognitive difficulty that mechanical difficulty in leveling would be wasted
I like leveling being challenging personally. The problem is our attempts to make leveling faster also made it easier.
I think this idea is worth expanding, especially when (and if) they decide to revamp BC zones.