Only one... you honestly want me to only pick one??? Well for me I think the most intriguing aspect, and the one that keeps me meticulously reading clues, notes, souveniers, plaques, Paragon Times, etc etc, is the massive arcs and overall story. Learning the horrible secret behind the Lost Mutations, Discovering the workings of the Internal Rikti Politics, Putting together the Pedestrian clues that Suggest Council ties to Arachnos, and then Jumping off into the Multiverse where new threats move against paragon every day. The story is such an engrossing element!
However, I also have to say that the Costumes are a huge pull. I have spent... well at this point probably DAYS creating costumes for myself and others. Themed, wierd, color coordinated... So much to do, so few costume slots, so few charachter slots. . . yeah I know there's a lot of both but it's still not enough