The vast majority of an MMORPG is contained in the experience of leveling. In the process of going from level one to max, you experience a huge amount of content, lots of which you probably never even see. If a game caps at level 60, and each zone occupies your time for 5 or 10 levels, then you're going to see a large amount of diverse content for a very long time. This to me is what defines a game.
The process of leveling is where all the fun is in a game done correctly. It is in leveling that we feel a part of a living virtual world, not in end game. End game always focuses itself on end game mechanics. It is always an afterthought.
If I feel that leveling is a chore, the game is a chore. It is one and the same.
The purpose of the next expansion is not to present you a challenge so that you can reach the next stage of endgame, but to present you with a continuation of the living virtual world you experienced while leveling previously.
In WoW I am now repeating the same content over and over again no matter how you look at it, for I am at endgame. If I rolled an alt, I would only experience the same content I leveled in before. I have spoiled it for myself.
Leveling is the superior MMO experience and it to me is what should be focused on beyond all else. My best memories come from leveling, by worst memories come from end game.