Why do we run instances so fast? I know it cannot be purely desire for the reward at the end, because I know that even when I have been motivated by loot, I have no always run quite so fast. Some of it is instance design and gear, that we can steamroll the faceroll mobs. But I suspect some of it is a terrible spiral.
If you have a lot of time to play, what do you do? Play a lot! You probably run a lot of instances and thanks to unlimited randoms, you can just keep going and going. Queues are fast, so you can easily run the same place 5 times in a day. Do that for a few days, or a few months, and yes, even the best instance ever will be incredibly boring. If instead you had less time and so ran the instance less, at the same real life passing of time, you'd be less sick of an instance than the person who played more. In fact, I think that it's not merely amount of times running an instance, but also density, that once a day for a week is not as boring as seven times in a day. This implies that less time could be more fun.
In other words, there is an optimal amount of time to play. Any more than that and repetition sets in and you hit diminishing returns in terms of in-game rewards. The first random gives frost, then mere triumph, which while still valuable, are clearly much less so. The farming loop is profitable, but if you are flying faster than they are respawning, then you're going to gradually deplete them and again hit diminishing returns.
I think this optimum amount of time has changed over time, with significant shifts with expansions or notable patches.
When it could take hours to get a group, travel to, and complete BRD or UBRS, then it fit well with a lot of time. Someone with one hour won't have enough time. Someone with five hours might find themselves unable to complete two runs, and therefore hitting diminishing returns, but the general trend is that WoW once had a longer optimal play time. Also supporting this was the higher travel times, epic flying is faster than a FP and we used to do a lot of cross-continent travel, combined with more farming being done outside, particularly reputations. Someone could more easily play for long periods of time without repetition, though many seemed to just go with repetition. But even still, a player with lots of time has lots of activities to do, and may take much longer to hit diminishing returns on any of them.
In BC the optimal time went down as instances and travel to them became shorter. The implementation of dailies did reduce the relative value of endless farming, but there did still remain special materials or items to farm, such as marks of kil'jaeden or motes.
Now in LK the optimal time is even shorter with closely backed daily hubs, short BGs (R.I.P. AV), and even shorter instances, particularly now that we're so overgeared, which is itself partly a product of the short instance cycle which fuels our insane levels of badge spending.
Of course this isn't purely a factor of the game. Some players enjoy the game itself more, especially early on in an expansion cycle, and some are less susceptible to repetition-induced burnout.