This has been an issue imo for a long time. I know this is an MMO and that other players are just part of the gig, but it's immensely frustrating that other players can advance MY quests. There's been a number of times where the quest objective is something like "kill x person," and before I can even get into a fight with them, my quest updates automatically because someone else in the area got to them first. That should never happen! Also frustrating when non-objective related but still unique quest related characters get killed and don't respawn for you.

Then I either have to move on without actually doing anything to achieve the quest progression or abandon and redo the quest to get the full experience.