Solid character progression, griping class rotations, interesting (read: dark sun, morrowind, the burning crusade) world design, challenging content, fun leveling experience, good storylines.
I'm very aware that most mmos barely scratch half of those marks nowadays, but ideally that'd be it.
Great, let's break that down. Keep in mind I haven't played Stormblood yet.
Solid character progression
Final Fantasy XIV uses a vocation system, meaning that you can cycle between classes. Each class is geared towards a specific role. Character progression is limited to going from the Basic Vocations to their Advanced versions. There's no real character building like you'd see in WoW's old talent trees.
You'll want to treat this as a multiplayer Action game moreso than as an RPG.
Its hard to comment on this right now because the game is going through a sort of overhaul/streamlining that made some classes fun and gutted others.
Its even harder for me since I was a healer and we don't have much in terms of rotations, more like a list of priorities. Playing an Astrologian was pretty fun. Drawing cards to custom tailor the perfect buff for each member of the party was an unique experience. I was also surprised that I had maybe 15 more important hotkeys than as a Priest in WoW Legion.
interesting (read: dark sun, morrowind, the burning crusade) world design
Its fairly uneven.
If you enjoy the artstyle, FFXIV offers a pretty comfortable ambience. The music is great and the environments are pretty.
Problem is that this kind of artstyle doesn't fit the story at all. Its hard to buy into death, carnage and despair when the land looks so pretty and everyone looks like a japanese pop idol. Well, maybe famine.
Speaking of story there's an abyss between the re-release storyline and the first expansion. The former is a shounen snoozefest where you kill the monster of the week in quick succession. You also interact with a lot of characters who were estabilished in 1.0 so its a bit jarring. The transition though twists everything around and Heavensward is a good story, despite some of the Final Fantasy cliches.
And, of course, speaking of which this is Final Fantasy: the Theme Park. The setting has its own identity, but worldbuilding is a series of references to other Final Fantasy games. Especially in terms of bosses.
FFXIV does the WoW thing where content is scaled to the player. There's the standard experience everyone goes through during the story, which is easy enough for parties without coordination to fulfill and there's 'Extreme mode'. I thought it was a rather adequate experience for two strange reasons:
FFXIV doesn't officially endorse addons and modifications like WoW does, so content is scaled towards the normal UI.
FFXIV's tempo is much slower than that of WoW, which allows for more crap to happen at once during encounters. Even with the reduced number of party members (4 for dungeons, 8 for individual boss fights, 3 groups of 8 for Raids).
It takes a while to get interesting because of the reduced number of abilities at lower levels. However, since FFXIV has on the fly level scaling it also adds incentives for players to do old content and pull new ones through the storyline. This means that you'll be doing dungeons and full party boss fights much before max level content. Things won't get monotonous since you'll be doing vastly different sorts of content even at the endgame. But since people's gear are going to be scaled down to the best equipment for that level, those low level raids and boss fights won't be very difficult and only the most basic of coordination is required.
What really got my attention in FFXIV is how mostly nice the community is. You'll get the occasional kid playing as the tank and fucking shit up but players are surprisingly civil when compared to those of other multiplayer games. Been called amazing more than once just for teaching people how to do boss fights. In WoW would get pissed that someone needs to be taught.