When is fun and how long does it last? I suspect this is a fundamental split in attitude which determines how people view WoW, and probably other activities as well.
After writing this I realized that the terms were a bit confusing, so I'll be repetitive. Temporary fun is that which you enjoy right now. You might log off and immediately forget what you were doing and the fun will gradually fade. In addition, when you log back in, the fun you had won't have made much difference. Imagine if you spent a night killing guards in Stormwind; no honor, no rep, no gold, no quests, nothing except the fun of killing. In contrast permanent fun is that which creates a persistent change. It could be a rep grind or getting a piece of loot. While these can become obsolete, they still remain as a permanent reward for the activity.
On one side is the temporary fun, it is right now. Fun is not something to be delayed or built up towards, ideally. Obviously in practice there will be moments which are not fun, such as logging in or loading screens. The goal for a developer should be to maximize instant and temporary fun, fun right now. If the player has a moment during which they are not having fun, the developer has failed. This doesn't mean that a game should be a constant ADD-driven festival of flashing lights and rewards at high intensity, but that at any given moment the player should be having fun. This can go up and down, but should avoid every being zero and should never be negative; with the opposite of fun being frustration, anger, and annoyance.
On the other side would be fun as a permanent thing; or to put it another way, fun is the additive process of everything before it. Fun is something to build towards. This may involve actually not having fun at many points. This draws upon the human tendency to more highly value those things which are preceded by that which they do not enjoy or that which they perceive as a challenge. A good example is a boss; rather than kill it in one go, take five tries. The first try will have positive fun, but it will likely go down from there, even into the realm of negative fun; however these low or negative fun moments will serve to increase the fun of the final kill. Consumables and reputation grinds would be more classic examples of building up to fun; they put the player through a potentially negative fun process which gradually moves towards positive as they get closer to the goal, peaking just before the goal is reached.
When looking at these two sides of fun, it is critical to avoid putting value descriptions on them. I don't mean that you can't try to put a number on them, like a rep grind will cause negative 10 fun followed by positive 15 or whatever; but moral values should be avoided. So temporary fun should not be described as mindless; this carries a negative label, in addition to being inaccurate. Temporary fun can be very conscious and active, such as solving a problem (assuming the person enjoys problem-solving). In contrast permanent fun can be very mindless (didn't I just say not to use this?), such as the example of rep grinds. Going even further though, mindlessness should not be undervalued for it can at times allow for an almost zen-like state of peace.
Since I love self-contradiction, I'm going to go ahead and claim that these are not opposing states, but instead are complementary. The success of WoW comes from the devs finding how to maximize the combination of the two, finding when a small loss of temporary can result in a lrger gain in permanent, or the reverse. For example, the reduction of true rep grinding (such as killing twilight cultists or skettis mobs) has given way to rep through instances, increasing the temporary fun of the process of the permanent fun. Another example is the reduction of consumable requirements, reducing the permanent fun of grinding mats in favor of the temporary fun of actually running a raid but with a gain of the permanent gaining of loot as well. Badge progression adds the permanent fun of item gains to the potentially temporary negative fun of running the same heroic for the hundredth time; or it might be adding permanent fun to the temporary fun of running that heroic that you really love.
So what's the relevance? For any given activity you can ask yourself about what sort of fun it is and whether it is worth it. If you're having temporary fun, compare that you other activities, especially IRL; so don't skip a night out with friends in favor of killing Stormwind guards (if you really prefer killing the guards, you need better friends). If you're not having temporary fun, make sure you're having permanent fun and then make sure it is worth it; keeping in mind that the relative values of fun are not fixed, so maybe now is the time to farm Stormwind rather than saronite; but later it will be the reverse. If you're not having temporary or permanent fun; do something else. This is especially important if you're leading others; so raid leaders, after the tenth wipe with no progress when everyone is frustrated, go kill something else.