GW2 version of it was piss poorly implemented. I do think it will be the next thing copied to all hell as opposed to grabbing 6 ! and going out to kill 5 of these, 4 of those, 8 of that and pick up 6 of this while your out there. Rinse repeat.
If done correctly you could have a living breathing world where each zone or area has a story or event arc that is always in some form of progression. That was GW2 biggest issue. Many zones were simply empty with nothing going on and everyone ran to the area that had 3 or 4 constant events and did just those for 5-10 levels moving on to the next area like that. This left a huge portion of the world empty and useless.
If the world actually felt alive, doesn't really have to be, the scripting just needs to be done so:
1. Events fire off even if there is no one there the NPC's should be fighting each other. NPC's need to be smart enough to bring more or be tougher the more people you have. They should also scale in a smart way so they don't get stuck. Solo guy isn't going to clear a camp full of elites when the group that was there bugged out so those need to either de-level or despawn after x time or Y deaths without progress.
2. Events need to be plentiful and varied to hide the "you're just doing the same thing over and over again". That means there should be enough events where it isn't the same old shit every single time. If you need 8 events to keep the zone hopping then there should be 12 -14 of them that get alternated through the system at varying times.
3. Events when completed need to have a progression so that the victors feel that they have won, yet when the world resets its not just a "yeah we won" and its all gone back to shit in 10 minutes when it resets. The event needs to progress in a realistic way.
Think Bandit cave (always here and respawn here first, think open world dungeon big), no one kills, they move out of the cave, no one kills, they take a village, fortify the village, take surrounding area, start impeding roads and traffic. Maybe merchants leave. Push right up to the main town areas until killed. People focus on this area, then another area will become the hot spot. Zones should be big enough where it should be impossible, even on launch days, to completely have the zone shut down.
Now these have to be done by 100 people and 1 person. That is where the problem lies and how that scaling adapts (how often it checks) how many people are involved. There also need to be a ton of those events in a large zone so that there is variety. Wolves, bandits, euridite 1necromancers, giants, trolls, dark elf assassins, Gnolls, orcs etc all with their little story in the area and reason for existing. These factions should also fight each other and behave in ways that are believable.
IMO that would be 100x better than the ! ? shit we have now. Where a developer pretty much just rams a finger in each nostril and drags you on THEIR pre planned trail through the game. Yeah its all the same we are just killing shit. I'd say its much improved over the hub to hub community destroying crap we have now. Throw in some mentoring, reasons for people to interact / communicate and just take my money.