Final Fantasy games tend to have beautifully realized worlds, nice visuals and a fun vibe in general. It translates well to an MMO.

Also, while everyone might not share this sentiment, I think forcing everyone to play through the story of every expansion is a good thing. I started playing this game during Shadowbringer, and my first instinct would've been to just level up and skip all the dated "irrelevant" content, like I would with most MMO's. It's what I did with Shadowlands trying to get back in WoW, and I lost interest pretty fast.

In FFXIV I feel invested in my character and the world, though, and it keeps me coming back. That said, this really isn't a "full time" MMO. For me it's a gap filler between other games and something I play around with at work.