So I just finished leveling my warrior from 85 to 90. This time, I tried what I'll call "the really long way." This involved fully questing out the starting area zones, and not moving on to the next zone until each was done. As a result, I ended up hitting 90 early on in Townlong Steppes rather than in Dread Wastes.
Interestingly enough, my played time from 85 to 90 for this particular character was almost exactly in line with all of my other characters, and actually toward the faster end of the range. The warrior came out with a total played time to 90 of 20.5 hours, which makes him just slightly slower than the hybrid level and a half method that I used with the rogue, who finished at 20 hours, and toward the low end of the range of the other characters who varied from 20.5 hours to 25 hours played.
So the definitive answer in terms of what the most effective and efficient leveling path to 90 is in MoP seems to be to do whatever the heck you want. It honestly doesn't matter.
Well played Blizzard. Well played.
I have one more character to level to 90 before starting on my Monk and that is my Shaman. He's getting pretty close to level 87 and is getting toward the latter portion of Valley of the Four Winds.
After that, it's going to be time to Monk things up!
Here are the played times for each of my characters that I've leveled 85-90 so far:
Warlock - 28 hours (started on launch day, so always slower)
Death Knight - 23 hours (changed as soon as leveled)
Druid - 24 hours (changed as soon as leveled)
Paladin - 25 hours (started as shockadin - probably slowed him down)
Hunter - 22 hours (changed as soon as leveled)
Rogue - 20 hours (hybrid - stay a few bars past level)
Mage - 25 hours (changed as soon as leveled)
Shadow priest - 20.5 hours (changed as soon as leveled)
Warrior - 20.5 hours (fully quested out each zone)