Well I really enjoy making hard quests and I don't mean hard cuz you have to kil 10k mobs and they take time I mean quests you have to involve brains in it, puzzle or gathering some kind of stuff, also I love making quests where I have to interact with the environment and with the npc's. Some kind of guidance to the place where the quest starts would be nice but not to all the things you need to gather only to the begining.
As for the mechanic of the quest system you can make more questgivers to be involved with one quest (complete this and this quest so you can complete this quest) , you can make quests which will take more time like you have to wait for a npc to come to town and assassinate it but it will come after 2-3 hours (real hours) or it comes with a ship the next day and you have to wait him 24 hours and kill him at the docks.
Something I absolutely loved in Guild Wars 2 were the story quests every 10 levels you get, these are special quests which follow your story (at the character creation you have some options to choose and create your character story and the quests are following that story) and the rewards are pretty nice. They also have cinematics in them the normal quests don't have.