I don't want to rain on your parade but there are a few things I'd like to mention.
Today it doesn't take as long to level as it did in the past. Back in the day it took infinitely longer to level 1-60 then it does today to level 1-90. Unlocking a dungeon was a major benchmark. Sometimes you spent weeks or months grinding Deadmines, Scarlet Monastery or Uldaman. Getting to from 30 to 40 was a massive achievement that took endless grinding. This situation persisted pretty much all the way until WotLK when Heirlooms and lowered mount costs and level requirements made alts a viable option.
While leveling today teaches you nothing, (you can pretty much afk dungeons, bot BGs or autorun quests 1-90) in the past it did teach you tons of stuff. One of them being patience. Like when you had to run 10 minutes back an forth on foot just to pick up and deliver a single quest that only rewarded 2% of your xp bar and was 5 levels under yours because it was in a zone that was 10 levels over the quest level you learned that 1, Dying was not fun (corpse run on foot at normal runspeed) 2, You preferred to go the long way round a group of mobs because it was safer.
What leveling used to teach you wasn't your classes abilities, but patience and situational awareness. Plus it gave you time to learn about things like addons, agro, other class abilities.
I'm not against the level 90 boost. I used it myself. What bothers me is the total and utter lack of a learning curve in the game at this moment. You can go 1 to 90 without learning anything about being a team player, about being aware about your environment or anything. You hit 90 (booster or not) being utterly unaware of anything, go into 5 man HCs and you stay utterly unaware of anything (let's be honest 5 mans are a joke), you go to Timeless Isle, running around doing nothing staying utterly unaware, then you step into LFR or Flex and is a total and utter disaster as both are totally out of your league. Unfortunatly you might get lucky in LFR get carried, again learn nothing, which means you are "ready" for Flex or Normals.
WoW needs some of type of a learning curve back in the game. One that works, is not utterly broken, and actually teaches people to play the damn game again.
Damn all you jsut said is SPOT ON!! This is the problem. One dont "really" learn your class by levelling 1-90 manually!