Blizzard has moved from making the endgame only accessible to the most high-level raiders to the exact opposite, making it accessible to nearly everyone. This mean even my mid-sized guild can nip into Ulduar and down the first couple of bosses with only a little fuss and a fair amount of effort. Yet the changes are not just limited to the endgame.
These accessibility changes now encompass the whole of Azeroth; from transportation to the amount of XP needed to level, continually lowering that learning curve to enable players to hit the endgame sooner rather than later.
Thanks to Recruit A Friend, your alts can join in the fun as well while your main gets kudos (and a groovy mount) for introducing a newbie to the wonders of Azeroth. With this comes the ability to grant a few levels. This begs the quesiton, why not just add an insta-level button? It would be nice wouldn't it? To just be able to stand in a city and ding dozens of times a la that video where a friend got their recruiter's alt to 60 in about twenty minutes.
However, it would also defeat the entire point of playing the game. As your toon moves through levels and you learn how to play your class, this is integral to learning the game whether it's your first toon or your fiftieth. After all how many people roll two of the same class?
On the PTR, mounts can be learned at a new-time low level of 20, a massive drop from the original 40 levels back in my day. There is, of course, a method to this madness. No more gauntlet-running through STV and being ganked by mean Alliance/Horde players who have nothing better to do that one-shot a much lower level player and dance on their corpse. This is basically the gist behind the whole idea; rather than speed up the leveling process by improving the XP gain or lowering the amount needed to ding (at least this time around), Blizzard has decided to literally speed up leveling by allowing you to get your mount another ten levels early.
All these changes point to Blizzard being smart. It's important to them that players learn their class, which is something you just can't do by going directly to level 80. Yes, you can learn how the world works and how to make money or train professions but you cannot just waltz in and play a capped-out character. Believe me, I tried. During the Wrath beta I got myself a Mage and headed to Northrend. As I quickly found out the class was completely alien to me, the talents meant nothing and to put it bluntly, I sucked.
Of course, at a more financial level, the other reason for leveling is in order to keep those subscriptions rolling in. It might only cost $14.99/12.99€/£8.99 a month to play, but multiply that by nearly 12 million players and that's an awful lot of money. Now factor in the number of years since WoW hit the market and you have some seriously large numbers. Added to this, once players have hit the level cap, this is where the 'mini-expansion' style patches come in. But here is where some players will still want something new and different during the lull between expansions or new patches and will reroll a second toon. Perhaps your guild needs more tanks or healers so you go back to the start and begin the leveling process again, learning a new class but benefiting from prior knowledge of quest hands in and money or XP-granting heirlooms from your main.
While I personally dislike leveling intently, it's only since hitting 80 (which I admit was a heck of a lot easier than 60-70) that I've come to appreciate the true wisdom behind this strange game mechanic and my friend's wise words.
It's not about torturing players, it's about teaching them how to play the game.
Leveling will prepare them for the raids to come, for 80 and the call of Naxx, the Eye of Eternity, Ulduar and inevitable Icecrown itself as well. Indeed in recent lore leveling could even be said to prepare us mentally and physically for our character to face the greatest challenge for any Azerothian adventurer thus far: the fall of the Lich King himself. This is why it will always remain a vital part of why we play WoW in the first place.