Sidequests, lack of meaningful rewards for those quests, lack of interesting stuff to find while you explore, etc etc.
The structure of the game outside of those linear action segments is so archaic and boring. So much of "go to next outpost, talk to the guy with the quest marker who dumps some info on you, talk to 3 other people who had quest markers pop up over their heads, go kill the bandits/monster/whatever, turn in quest, talk to the next guy with the quest marker that just popped up on his head…."
The sidequests are generic MMO stuff and there's tons of them. Most of them reward you with crafting materials that you have literally hundreds of, and nothing to use them on. Every new weapon/armor is just a minute stat increase over the previous one.
The exploration zones mostly have nothing to find except shiny dots that give you like 3 Gil or some useless crafting item.
Even lots of the story sequences are bad. For every riveting GoT style drama scene there are 10 others where the characters just stand there like mannequins yapping on and on about something nobody cares about.
Imagine a very pretty but shallow DMC clone, but in between each stage you have to do like 6 hours of FF XIV A Realm Reborn drudgery to unlock the next stage. That's XVI in a nutshell.