I have been asked many times what has made WoW stand out from all the other MMOs out there. The answer itself to me is not a difficult one. Warcraft's lore and story is what keeps people playing this game. Sure there is side things to do, but those who are interested in those come and go. Think about it for a second you may not read the novels or pay attention to quests, but say when a guildie starts talking and explaining what we are really doing and what you are dealing with you listen.
In WoW there are these things and those times we always remember. It could be a questline that you love doing because of the story. Many have told me that the Thrall and Aggra Elemental Bonds questline is one of those quests. They felt like they contributed to the lore of the game themselves they feel like they are directly part of the story. Other games don't have quests like this where you are developing in the story itself. You are very disconnected you are there are told to go kill these things, collect those things, and then come back to me. Yes WoW still has some of these quests, but even the way they are done is much different than even classic WoW.
This is the point when you are involved you remember. The only way you are going to remember a quest where you are told to go kill something or collect something is if it was so rare it took forever, the quest itself was bugged, or was hard. You aren't going to remember to them because of Dave and his epic quest giving skills. This is Blizzard's theory they want you to remember quests, raids, and everything else because of the story involved in everything. Think about it when you remember you can talk with others about it, and if they haven't done it they will want to. It is marketing at its best word of mouth. Of course, this can backfire in the same way, but Blizzard has seemed to avoid this for the most part.
So if I had any advice for new MMOs or even to Blizzard in WoW it is simple. Make sure that you use the story of what you are involved in to ensure that the player remembers playing it and it was a fun and enjoyable experience.