*As much as the beginning
Don't get me wrong, raiding can be fun along with level-capped PvP. But leveling just has something more to it. Maybe it's the way you don't look at an instance and say "eh, no gear there, skip it" instead you go because so often it's more fun to play in a group and the XP can be better (especially since it's more fun, so you don't get bored with grinding). You don't outgear instances, you outlevel them, and then go on to another set. There are 2-4 instances to do at a time, constantly passing one and moving into another. Once you are done with heroics you have a decent set of three raids, though two are pretty small and one of those contains Malygos phase 3. Naxx even has four starting wings, so you have choices of what to do when learning. After that: Ulduar. Just Ulduar. Grind out your reputations and gold for consumables and stuff to stick on new gear.
I've found this too while playing Civilization. I enjoy the early growth and development of my civ. Learning new technologies and tile improvements seems parallel to new spells. That first war is a lot of fun. Then eventually I look at the map and think "Okay, I'm winning, but if I want to win I'm going to need to either wait for the space part technologies or I have a hundred cities to conquer." It turns into a grind. The dynamic feel of the early game is lost.
Another game where I ran into this was called Escape Velocity. It's a fairly old game by now, but still a lot of fun (enough for them to make two 'sequels'). I recommend it if you're looking for some idle fun. You'd start out weak, poor, and totally ignorant of the universe (empty map). Eventually through story lines and trading and missions and combat you'd find yourself having 'won' the game (completed a major line) and there's not much else to do but conquer the universe. This is done planet by planet, taking down hundreds or thousands of enemies.
However there are two ways that I've found fun in end games. First, there's PvP (practically the point of any FPS). Second, there are player mods, EV is very very adaptable and there even exist some total universe changes which are practically entire new games. I suppose this are the same in a way: other players make games remain fresh longer. Why doesn't that work as well with WoW?
This is a common theme in life: The process is more fun than the result.