Lagrely the story and world is what drew me in initially since I've been a fan since WC2: Tides of Darkness.
That said, I know many people aren't in it for that. As such I'll try to give you some reasons I've gleaned from playing both it, and other MMOs. Rift and Wildstar in particular. Both do certain things better than WoW does, but there's also the flipside where WoW does some things better than they do.
I think the biggest thing WoW does better than the other two right now is questing. I don't much mind the grind and the few quest types. That happens in every MMO. The trick that designers have to learn, I feel, is how do you make me CARE enough to complete the grind and thus the quests? The countless weird little quests that reward you with story-related things or moments of humor kept me going and kept me interested. Things like The Day Deathwing Came, the Rhea's Egg questline, even the little one-off quest in Hillsbrad Foothills for the Forsaken where they turn things on their head and have YOU become the quest-giver to various NPCs that represent stereotypes of WoW and MMO players in general. The starting chains for the Goblin, Worgen, and even to an extent the Pandaren are interesting as well. Though frankly I think they did a better job selling me on the experience of the first two over the Pandaren.
There's also the aspect of friends. Despite quitting for over a year I still find there's players around who are STILL playing and it's good to see them. It's good to be able to catch up and hang out with them when I can. Sadly those people are becoming far too few for me so odds are when Wildstar launches I'll be moving from Azeroth to Nexus. The basic thing is just that MMOs are better with people to share it with. Guild Wars 2 is probably the only MMO I know of that has a questing experience that can truly satisfy solo players
Ultimately I think it's a matter of you're seeing what you expect to see. The MMO market right now suffers from a lot of that. I've seen it in the Wildstar beta a lot. People come in and instead of walking in with the mindset of wanting to have fun and just trying to lose themselves in the universe and story, they come in utterly jaded and impatient and frustrated with everything. Everything is a clone of WoW, or ESO did it better, or this, or that, and where's the VA work for every quest like there was in TOR? Why is PvP just capture the flag again? What kind of lazy philistines made this game?! Ugh! You people sicken my highly-evolved (read: picky and unable to be pleased) palate!
One thing in the above people's defense, though, is that MMOs tend to have....well, Yahtzee pointed it out in Half-Life and other games as "choose your level of immersion". You can either just click click click and get the quests and do them rapidfire, not caring about how or why things are being done unless they bludgeon you over the head with it, OR you can take a moment, slow down, and actually READ the quest text. Either one works depending on the goals that you have for playing the game, but it's understandable to think that a game has no immersion when we're so used to having it forced on us in other genres where worlds are dripping with flavor and story and immersion and then a game decides that it's gonna put it in, and you can take it or leave it as you like.
EDIT: for those people who are curious, TotalBiscuit appears to agree with me about the one-off quest as he made a video showing it:
http://www.youtube.com/watch?v=VvF1o1ObPr0
Just if people are curious and want a bit of a laugh to start their day.