The story, the fact that it feels like final fantasy (in a different way than XI did), the nostalgia, the fan service. The great community (despite some of the crazy stuff you see around these parts, lol) and just the fact the game is flat out fun. There isn't a counterpart in the MMO market that gives me as much fun for the time spent in game. Also the accessibility of picking up the game and doing something. Whether it's switching to another class to finish getting all the needed cross class skills, grinding fates for an event/relic, jumping in a roulette, etc. it's all fast to pick up and do something. While my inner 15 year old grumbles about it being too "carebear" the rest of me is thankful for how quick it is to find something to do. I loved camping HNMs and all, but I don't think I'd have time for end game activities of that style now in my early 30s.

My favorite part of the FF MMOs (XI, XIV & ARR) from a core game concept viewpoint has been the fact that they keep the job change concept that is so core to the FF experience. You don't have to go create a new character if you want to try a different class; just change weapons.