The secret world is the most interesting to me, since I think secrets, mysteries, and discoveries are the most exciting thing in an MMO. Unlike single player games, where people post all the secrets online right away, MMO players tend to guard their discoveries, because knowledge other players lack is power, and because the developers can never be trusted (nerfs). Secrets lead to a need for trust, and trust leads to friendships; although there will always be betrayal as well.