Many quests still can't be completed because...

[Zentoth]

... you have to find an NPC that's not there
... you have to interact with something that can't yet be interacted with, e.g. using the Taxi Stand
... there is no loot, yet, which may include quest items dropping from kills
... the the status of the objective can't change due to a missing auto-completion step, e.g. entering a designated area
... accepting quests no longer works, possibly due to a quest in the quest log having an auto-complete pending