I have played a few MMO's since I started back in 2003. From each MMO there was something I enjoyed even though there were a few I did not enjoy. The one thing that stands out the most was player created content. Players were able to log in to a instance creator and they were able to cycle through what they wanted it to look like. They could add NPCs and control the dialogue choices.
I think that having the option to allow players to create a small instance would be difficult because of the limit to locations to travel from but I think that overall it would be nice to have. The complaints of not enough quests or too grindy would go away if we had the option to go into this added material. It could alleviate some of the strain on the developers from having to deliver new material every few weeks. One thing I have read on the forums was people's want for more Shivering Isles quests or detail. This could be something the players themselves come up with.