I always complete an area's quest before moving on and lately I am coming across with extraordinary amounts of quests that do not progress.
From uninteractable NPCs to turn the quest in to NPCs that don't give their supposed quests.

For most of these issues I created a bugtracker, but none are resolved.

An example is the PvP quest at Hellfire. There is a chance the NPC will never give that quest, even if you have done everything in the zone. You can get that quest right away, without anything in prior.

The crucial problem with Hellfire's PvP quest is that you cannot get the zone's Lore Master without completing that quest. It isn't actually completing X amount of quests within Hellfire; the PvP quest must be completed to get the achievement.

I would say close to half of my characters had that Hellfire bug.