I want to tell you two stories about people who decide to try WoW.
Alex has decided to try out this popular game called World of Warcraft. He installs it, boots the game up, and starts exploring. He has little idea of what he's doing but hey, there's a big world out there to discover. He doesn't realize that doing grey quests is bad experience but he doesn't care either - he's having fun seeing all this new stuff. He then discovers this dungeon finder thing and sees this group stuff with awesome loot - that's BLUE. Whoa. Then he goes back to being 35 and questing in Duskwood (yes, I have seen that exact thing happen). Eventually he starts to pick up things though he's still generally pretty clueless.
Despite the fact that he's learning, leveling is slow in general and it takes him something like 300+ hours to hit maximum level. Assuming he plays, say, 10ish hours a week, that means it takes him seven and a half months to hit maximum level.
He's enjoyed this process - he's made steady progress toward this goal of maximum level and he's explored a bunch of the world while doing so. He never particularly wanted to level faster since he had no particular reason to be maximum level. He's about to discover raids and organized PvP but that was never even on his radar before. He's generally happy.
Bob has a friend (call him Chris) who plays this popular game called World of Warcraft. Chris eagerly describes working as part of a big team to defeat huge dragons and giants and such or maybe tells him about pitched battlefields between two warring factions. Bob decides this sounds cool and wants to play the game with Chris so he picks up WoW. However, he now realizes there's a bit of a problem - Chris is maximum level while Bob is level 1 and can't do anything with Chris.
Bob asks Chris if he wants to make a new character to level with Bob. Both of them only have 10ish hours per week to play, however, and Alex has scheduled raids along with weekly chores to deal with. Thus, Chris can only play a few hours per week at most with Bob - meaning that Bob will quickly outlevel him and the whole idea of playing together at the same level will be moot. Alternatively, Bob can simply only play a 2-3 hours per week with Chris and not even play WoW with his remaining spare time - which also means it'll take Bob even LONGER to level and do all the cool stuff with Chris at maximum level.
I think everyone will agree that Bob feeling like he can only play WoW whenever Chris is NOT busy with raids/chores/whatever is a bad idea. And arguing that Bob should make multiple characters to avoid this issue seems a bit silly - but if anyone seriously thinks that is a good idea I can go into more depth on why it is not.
So let's assume Bob simply levels as fast as he can without Chris so he can join Chris at maximum level as soon as possible. Bob has Chris helping him with game information so he manages to level twice as fast as Alex. At 10ish hours a week, this means Bob will hit maximum level in 15 weeks (150 hours needed due to him being faster) - which is still nearly four months.
That's Messed Up
Let's look at that last sentence more closely. Chris convinces Bob to try this new game and when Bob asks Chris when he can do the raids and battlegrounds and such with Chris, he gets told that it will take nearly four months. Four months of playing one character to even get to maximum level (I'm not even including gearing up at max level and learning how to really play the game - let's assume those magically happen instantly). Four months before he can play with his friend.
That's not a very friend friendly game. You can jump right into a strategy game like Starcraft II or Warcraft III - your friend will be terrible but you can still play. You can hop into a game like Halo or Call of Duty - ditto. Or something like DotA or League of Legends - your friend may have less heroes available or may be missing some small bonuses, but they can still play with you.
But not in WoW. In WoW, by default, it will take a person playing 10ish hours a week about four months to even be able to play with a friend who lured them into the game.
Between a Rock and a Hard Place
We can see here that we have two very different kinds of new players (probably more, but these two at a minimum). One is here to explore this new world and is fine with slow leveling - he's just seeing what's out there and has no idea what he's "missing" at maximum level. He's in no rush to level and is content subscribing while playing the game.
But the second wants to play with his friends who are all max level. He might enjoy leveling a character and exploring the world and such at one point, but right now he simply wants to be able to do the stuff his friends tell him about. And his friends probably don't want to have to make a new character every time they convince a new friend to try WoW. So this new guy wants to get to maximum level as soon as possible.
When I hear people complain about how leveling is too fast in Wow, this is what I think about. The fact that an incredibly long leveling game works well for some players but is horrible for people trying to get friends into the game. That's why I think Blizzard is trying to err on the side of leveling being too fast - by introducing Recruit-A-Friend and now the 100% experience bonus.
And even Recruit-A-Friend has issues because, again, the two friends have to play together. And if player A is busy raiding and doing weekly LFR/Charm/Valor chores, he might not have the time to play with player B very much (if they were both maximum level they could do it together). Being told "you have to spend a week or two building your character before you can play with your friend" sounds a lot better than "you have to spend four months building your character before you can play with your friend."
So Where Next?
This does seem to lead to an interesting conclusion - perhaps it's better to really minimize the idea of leveling - if we think most new players want to play with friends instead of just picking up the game on their own (which seems to be more and more likely these days). Instead of it taking up to 4+ hours a level, make it take like a maximum of 60 minutes a level and much faster initially. Make lots of questing out in the world something to do at maximum level (and give charms/valor like dailies) rather than something everyone has to do to level. While leveling, give players a taste of what is going on in a zone and make leveling a whirlwind tour of the zones - then a player can go back at maximum level to explore the zone and find its lore.
This obviously raises a number of issues, however. Rewards being one of the main ones - traditionally quests have generally given worse rewards than initial (heroic) dungeons. Blizzard has noted time and time again how players will use the most efficient way to get their loot. So if quests still reward 437 greens, you'd expect players to generally not do them at maximum level because they're getting 463 blues from dungeons. But perhaps they could give 10 valor and 3 charms each or something.
Imagine an MoP where each level only took 30-60 minutes with the main plot of each zone set up and then once you hit 90 you could do all of the quests in the various zones to see the plot. There's your stuff to do out in the world at maximum level.
Right now it feels like WoW is trying to act like SC2/LoL/etc at maximum level but is also trying to act like a traditional RPG storyline with a massive leveling grind. A playthrough of a game like Mass Effect (2/3) probably takes 30-60 hours maximum - and that's considered a full game. WoW leveling takes far longer than that and you have to finish it to do the "main" content of the game. Maybe it's time to vastly de-emphasize the idea of leveling and make questing in the world something you do at maximum level.
I'm sort of rambling now, just trying to sort though this idea and playing out the various scenarios - it would be a large change but it makes me wonder. Instead of the question being "Is leveling too fast?" perhaps we should be asking "Is leveling too slow?"