A simple question, but apparently one that isn't easily answered. Apparently ffxiv isn't doing that job well at all, but some aspects have people coming back day after day.
Long before MMO's i played on MUDs, back when there actually used to be a ton. New ones would pop up daily each based of a few basic designs with some standard and some heavily customized. Some with 10 players, others with 1000's. Toward the end of my MUDing days i even did a bit of implementing but thats another story.
The MUDs i played i would play with friends. And first and foremost that is what kept me somewhere. Secondly i loved a challenge. I'd always pick a MUD that offered multiclassing. With my favorite being Intrepid, where you picked a class and every 10 levels you could multiclass down to level 1 again with the abilities of both classes, but your TNL doubled each time you did this. It make your character incredibly stronger, but took longer and longer to level. It was a great concept as you could go warrior, then multiclass as priest and basically be a paladin, or blm for dark knight... ect..
I knew a guy who was a warrior, and never multi classed. He hit level cap (think it was 60, its been a while) and formed a guild. Eventually i joined however i had about 5/6 classes. When we got into a fight with another guild.. he would get destroyed by a level 20 multi classed character. Eventually it sort of made him rage quit. But i digress.
I play games for a challenge, and to occupy my time. I view MMO's as most view life. Its not about the end, its about the journey. I don't want to play a game where i can cap my level in a few months, i want a game thats challenging. Hell, i've beaten boarderlands so many times its not funny, but that doesn't stop me and my friends playing it again always having fake connections to have the mobs spawn harder and skip missions to avoid leveling to fast and getting ahead of the mob levels.
This is why i enjoyed FFXI, hated wow/warhammer/guildwars/everything else. FFXI basically forced interaction so you talked with people, made friends. Wanted to play with those friends. Was it too hard, not generally (though some of the CoP missions were a pain), but it took you time. The combat wasn't terribly fast paced, but it allowed you to talk to your friends and enjoy the world with them.
I want a diverse crafting/gathering system. I don't mind if its all dependent on each other, but make an auction house so goods are easily searched and found. Have story lines that shoot out everywhere, have a point to the path you choose.
I want classes that are unique and each have their strengths and weaknesses. Promote a sense of pride for being a class, for developing your character the way you choose. Make gear have requirements, so you have something to work towards. Have a wide selection of gear/armor/spells.
Oh yeah, player home/storage. What is the point of living in a world if all you have is your eq and inventory... talk about a bum life.