Quests, all quests should be shareable with your friends whom you're playing with. One of the core concept behind MMO is to be able to have a good time within a fantasy world with your friends as you adventure together, and you should be able to share all your missions.

Sharing is not just picking up the same quest from the same npc at the same time. Sharing means there's a share button for all quests, which allows you and those partied with you to see the quest areas as well as gain completion together and reward together. Even your story missions should be shareable with your companions as a guest or helper, with progression and reward customised so that only the character who initiated the mission will receive it.

Bottom line is I really believe that a majority of people play MMO because they enjoy being able to progress with their friends, and the system as of now does not properly support that demand.