now i know the debate in favor is saying that "well if you have a 90 already you have already experienced it", and i dont disagree with that statement but i dont fully back it either, its not till you have hammered through the quests twice i would say until you really experience them,
with that being said i bring up one important point that i dont believe any others have spoken about,
the terrain in pandaria is quite different than the previous zones, in particular the excess of ridges and un-climbable hills, i can't count how many times i would finish quests and be ready to turn them in, i knew were on the map the quest could be turned in, but 9 times out of 10 i would never be able to just run to the npc's location, i would be sent through a labyrinth of ridges and hills to get to the npc, and most times i would get lost and end up riding 3/4's of the way around a town to find the damn entrance.
because of the different terrain from previous expansions i would have to make a guess that it has made leveling time for myself increased by and easy 20%, and thats not questing, or killing mobs or collecting items, its just and increase in traveling
in my opinion i believe that if "tome of pandaria flying" isn't and option, atleast increase the angle we can climb hills with our ground mounts so that we aren't constantly getting frustrated because we got lost in the terrain again and have no indication on the map were we can travel to get around