I have leveled two chars from 20 to 100 and a few other from 7x+ to 100, and I can say it's a nightmare if there were not some cheesing mechanisms known by now (heroic instances, dungeon boosting). Even with cheesing it takes about double as long as it did before, and several seem to be
hotfixed today.
Especially 60-80 is pure horror if done questing, you can complete entire zones and overall get 3-4 levels for 5h+.
1-60 is fine, it has a nice pace and the revamped zones are not too long to quest throgh.
XP gains in BC/Wotlk should be buffed by 100%, questing there takes double as long per level as in any other zone.
80-90 again has the right pace (you need to do around 2.5 zones, and collecting pandaria treasures is also a nice boost thanks to flying), and WoD is as fast as it ever was thanks to flying, bonus objectives and treasures.
Scaling is a great thing, and since heirlooms no longer provide the power they did before you can fully take the scenic route by not equipping them. The XP bonus on heirlooms instead should be buffed so people can freely choose if they want to "enjoy" the content or just "rush" through.