But when is it done?
The definition of instance completion has changed over time. Let's see how.
To start off, I'm going to define two initial mindsets for how an individual will decide that an instance is done. The first is selfish and the other is cooperative. The selfish person will define the instance as done for them while the cooperative will define it as done for the group. Keep in mind that these do not necessarily result in different final definitions; the separate paths may converge again, or diverge ever further.
What makes an instance done?
At the furthest extreme, every single mob is dead, every gear has been clicked, and until there are respawns there is nothing to do beside run around. I've only seen this happen intentionally a few times, but some instances will have it happen naturally due to narrow, single paths which force players to fight every pack.
Then there are the middle grounds of every boss dead or every quest completed. Depending on the instance one may require more time than the other, so I cannot place them relative to each other.
Toward the doing less end is the instance completion message from the dungeon finder. Done, here's your other loot and bonus xp.
And finally at the lowest end is the instance achievement. This will be different from the instance completion in winged instances such as BRD or Maraudon.
Coming after finally (because I can't figure out where to fit it in) is the specific item goal. A player or group runs for a specific drop and once that's done, they're done. Or if it fails to drop, possibly the same, followed by a requeue. This is the guy who keeps leaving halfway into HoR.
Note that we didn't always have all of these criteria. Dungeon finder rewards are more recent than achievements which are more recent than quest completion or boss kills. And this is where the change has come from.
Whether the actual mindset has shifted from cooperative to selfish, the behavior has given the appearance of such a shift. A cooperative player on a quest goal can understand that others on quest goals may have other quests, so his quest completion does not mean the instance is done. In contrast a selfish player will define the instance as done when their own quests are done. However with the addition of various single-event announcements, such as an achievement for killing the one last boss or loot bag for the same, players have tended towards a thought of "I'm done, so they must be done too." This creates the same behavior, leaving, as if they were operating with a selfish mindset.
The selfish mindset isn't new. Long before HoR I saw players drop in the middle of instances after getting, or failing to get, the loot they wanted. But there was less of a tendency to assume that "I'm done, so they must be done too" Why? Because we knew from the beginning that our goals didn't always fit the perfectly linear path of a modern instance. Maybe it was BRD and we needed to kill 10 of a trash mob, but a path straight to the bosses would only yield 7 or 8. Or we're running into Maraudon and someone needs the crystals from the Khans, but they're not right at the instance entrances, so we'll deviate from the course slightly. Sometimes the quest mob isn't even normally present, but is part of a triggered event. From the start there's the idea if "I'm doing something different, so maybe they are as well".
But there was a more important factor still: nothing told us when we were done. Players had to define for themselves and their group when an instance was done. So if a group wanted to skip half the instance and only kill the third to last boss, they're done.
Now we have something to tell us when we are done. This doesn't necessarily align with how we've defined it. But it's the official "Blizzard says we're done" message, so it must be true and universal. This message is why Maraudon is missing two bosses. They're still there, but the "you're done" message comes before them, so players assume the individual and the group are done.
As the stimuli have rewarded different behaviors, so have the behaviors changed. Maybe we're not more selfish than we once were, but we're less thoughtful, assuming that what we want is what others want. And then to end on a tangent: humans are less kind to anonymous nobodies, so regardless of the change in stimuli, running with cross-realm people who we'll never see again will cause more selfish behavior. Add to that the simulated selfishness and you have the new way to run instances: alone.