Only read the OP, not the many... many replies.
I think leveling is both too fast and too long. It's too fast because you level so quickly, especially in the low levels, that gaining a level doesn't feel special, and you outlevel zones halfway through. It's too long because it still takes 60 or more hours to get a character to level cap unless you're extremely efficient and experienced with the game. It does not take 50 hours to learn a class, especially when most of those hours are spent in content that is ridiculously trivial in difficulty, and often played missing key abilities. Player power has scaled so much at level that I do more personal DPS on a typical level 70 character than an entire party in leveling gear did in Wrath. You don't learn anything useful doing that--- you don't have fights long enough to learn your rotation, you never need to use utility spells, and the incoming damage is trivial.
If they ever revamp the leveling experience again, my off the cuff suggestion would be for Blizzard to go back and adjust the levels. Drop the level cap back down to 60 (with or without a stat squish at the same time; with no squish a new level 55 has the stats of a current level 90) and, at the same time, reduce the total XP needed to hit cap by 33% or so of what it is for 1-90, and make the existing questing experience overlap a lot more to provide more options. For example, make vanilla zones get you high enough level to hit either Cataclysm or Mists quests upon finishing, say, Blasted Lands or Sithilus, with Outland and Northrend as alternatives to what are currently the 30-60 questing zones. Adjust the classic raids to be, for example, at level 30 for vanilla, then 40, 45, 50, and 55 for the other expansions, with the new raids at level 60. That way, each individual level is longer, but the total time to level is shorter and you can do entire zones without outleveling them.
At the same time, they should either retune low level characters or the enemies, especially in dungeons, to make the game closer to the intended difficulty. I don't want Cata hard for leveling content-- it does need to be easy at first so newbies can get their feet. I just want bosses to last more than 15 seconds and tanks to be permitted to tank properly (instead of the current 'lol ezmode' that has all the DPS pulling what they feel like and never dying.)