The best feature of Age of Conan for me up to now is the destiny quest line, which starts at level 5 when you first reach Tortage, and ends at level 20 with you leaving the island and entering the big world. I haven't done that last part yet, but only because you can't start the final battle for Tortage before level 19. I've done all the previous parts (except for the one bugged one), and received a rare (blue) quality robe as reward.
The destiny quests can only be played in nighttime mode, which is solo. But because it is instanced just for you, your destiny quest line depends on your class, or rather your archetype. Warriors, priests, mages, and rogues get 4 different quest lines. But the really fun thing is that the 4 quest lines all tell the same story, but from different angles. Some quests are the same, like for example every class has to fight through the volcano instance to talk to the slave master. But other parts of the quest are class specific. The rogues have to sneak and steal stuff, the mages have to become apprentice of the evil sorceress to learn her plans and foil them, and so on. And quite often the quest givers tell you that somebody else will do one part, and you are charged to do another part. And then when you replay the game with another class, suddenly it is you doing that first part, and your told somebody else is doing the other part. So basically you need to play all 4 archetypes to see the story from all angles. It's pure Rashomon, for the Akira Kurosawa fans out there, a great way to tell a story.
But even if you just play one part of the story, the destiny quest line is great fun. It is a coherrent story line, explaining what is going on in Tortage, and enabling you to vanquish the evil tyrant. By shipping you off to the mainland at the end of the story, the game nicely gets around the inconvenient fact that the evil tyrant will of course still be there for the next player once you beat him. It creates a good illusion of you really having changed the game world. What you have to do in the quests isn't that special, talk to this guy, slay that guy, etc., but the way the story is told is far superior to normal quests. You actually read the quest texts because the story isn't as random as regular quests, and there are cutscenes for the big events. I liked the destiny quests in AoC even better than the lore book quests in LotRO, where the multiplayer nature often made people hurry through them and not have time for the story.
Most of the normal quests in daytime, multiplayer Tortage aren't bad either. There are some bad apples, like a quest to kill 40 crocodiles of which there are far too few. But there are plenty of quests, far more than you'd need to level to 20, and so you can just skip the bad ones. What could be better is the placement of quest givers. Quest NPCs are placed all over the city and island, and it is far too easy to receive a quest, march off to an instanced quest location to do it, come back, and then find a quest NPC sending you back to kill the same mob in the same instance hidden in a different corner. So better run around Tortage and pick up all quests first, which then are conveniently sorted in your quest log by location, before going off to places like the White Sands Island to do all the quests there at once.
You could theoretically level up from 1 to 20 just doing daytime quests, and then skip the destiny quests, but why would you? I can only advise you to always first do the nighttime destiny quest line, and only switch to daytime when you need a higher level to continue the destiny quests. There only being one starting area from level 1 to 20 is certainly a disadvantage to replayability of Age of Conan, but I feel that the 4 interwoven destiny quest lines really make the best out that limitation. Well done, Funcom!