I could probably write a book on my opinions on what is wrong. I'll just pick 2:

Netcode - Honestly, Not really much of a need to go into detail. Look at nearly any other AAA MMO today (not even MMORPG, just MMO) and you'll see a staggering difference in quality of responsiveness.

Derivative content design - I've said this elsewhere, but I'd much rather have 5 different rides that are deep, well designed, and offer robust gameplay options for VARIOUS playertypes. Compare that to the existing design paradigm where you have 10 rides, that are as shallow as a puddle (no depth) and are catered to various niche player types. This leads to DOA content because if that niche doesn't like what you designed for it, NO ONE ELSE is likely to touch it either.

I've given countless examples over the years about how they could have made Chocobo Racing a real thing instead of the mess we got. Several examples of how Eureka could have been more engaging and involved for multiple playertypes. I've covered how LoV or Squadrons could have been etc.