If any two philosophers would have been excellent game designers, they would be Aristotle and St. Thomas Aquinas.
Aristotle called something virtuous when it performed the purpose of its design well. A knife, for example, is virtuous if it cuts well. You could choose to use a knife as a hammer, but it probably wouldn't perform that function well. There are limitations inherent in the design, though few designs are absolutely limited. A knife may sometimes act as a pretty good flat-head screwdriver, for example.
St. Thomas Aquinas believed that there are three things any person must know in order to be happy and fulfilled: (1) he must know the world for what it is; (2) he must know his own place, his role, in the world's design; and (3) he must know how to fulfill that role through momentary actions. If a person does not know how to translate his knowledge of the role into definite actions, his ability to experience the joy of fulfillment is stunted.
On the other hand, nobody likes to feel like just a cog in the machine. In every MMO game I play (and I've played or tested almost a dozen now), I inevitably hear some players criticizing others for not knowing how to play a particular group role. If these criticisms were general, like asserting that a defender should be apt at protecting his allies, then I would agree. But they are more often suggestions that a very particular tactical blueprint should be followed without deviation; thereby ensuring, in combination with everyone else's particular blueprint, that the factory of combat produces the agreeable (if bland) result predicted.
THE GIST OF IT
For most gamers, fun is best achieved through a balance of predetermination and free will. What MMOs have shown me is that stale routines can lead to an illusion of predetermination and confinement.
Consider real warfare. A Roman swordsman knew what his initial actions must be in order to preserve the integrity of the group's strategy (a phalanx formation). In order for the formation to be effective, each soldier must move toward the enemy at an equal pace, maintaining a position in the battalion's order, until he the melee combat begins. But... once that combat begins, the soldier must respond to the spontaneous (though trained) actions of the enemy and to the progression of events surrounding his immediate opponent (like the allied soldier by his side falling to an enemy attack). Likewise, in modern warfare, the soldier has a general place in unit operations, but he must respond to spontaneous events in a fluid and often creative way (here, "creative" often means applying his training to an unfamiliar scenario).
If an encounter does not seem spontaneous in its unfolding, if the enemy does nothing that vaguely surprises the combatant, then the encounter leaves an impression of being fated, of unfolding as it was certain to unfold. Utter predictability acts as an adversary to a feeling of free will. If something always happens, then it must happen. This is a basic assumption of the empirical philosophy which popular culture has accepted from modern science.
The problem of the infamous "grind" isn't that the player must struggle through an endless series of encounters to grow and realize his or her goals. The problem is the predictability of these encounters, and the player's consequent impression that his actions were never truly his own, but rather just the fulfilling of an external plan.