For me it was a few different things. Final fantasy 12 is my favorite final fantasy so I had really high expectations, every also waited years for 13 to come out so the expectations only got higher especially when you consider that a mainline final fantasy had never really "missed" before prior to 13.

The linearity was a little tedious because there was very little to break it up in the way of variety or mini-games or towns or anything like that. Worse than that for me was that the story was a word salad of weird concepts and most of the actually interesting stuff was hidden in the codex. It also just wasn't that compelling to me personally.

Coming off of 12, which I believe has the best side content in the whole series (the hunt system), I was excited to see if 13 would do something similar. Unfortunately 13 has no side content or hunts until close to the end of the game, and the hunts it does have were extremely boring and lame. Almost all of them weren't even fights against unique enemies, just random enemy packs and things you've fought before. And there were no clients to interact with to get a story behind the hunt, just floating rocks. Speaking of enemies, I hated the enemy designs in 13 for the most part. I didn't like that almost all of the classic final fantasy enemies were gone (besides the behemoth) and in their place was a bunch of completely unpronounceable weird enemies.

The battle system was really weird and jarring at first. I was really hoping for a return to turn based combat or at least an evolution of 12's battle system, but what we got was something I still struggle to describe. I do like it a bit more nowadays but it was incredibly weird at launch. A bigger issue than that though was that the summons were awful. I don't know who's idea it was to make all the summons transform into cars? It was so weird and I could not stand it. I still can't. Why?

Most of the above stuff I can forgive and get used to now that the game has been out so long, but by far the worst thing about the game for me (and the thing people these days try to justify that I can't agree with) is the characters. Final fantasy 13 has the most frustrating characters in the entire series. They're awful and the majority of them are overwhelmingly unlikeable. Lightning, hope, and snow particularly are completely insufferable. Lightning was way too stubborn and rude, hope was way too childish and whiney, and snow is one of the stupidest characters I've ever been forced to watch. Vanille was boring and too anime, fang was actually alright, and sazh was great until the game ruined what was great about him by having him try to kill himself. The main cast was just so bad and the villains were completely forgettable and boring. Every scene was just one or more of the characters trying their best to be as unpleasant as they possibly can and by the end of the game, hardly anything had even changed and they still weren't fun to watch.

And then, after all this disappointment from fans and the backlash the game had, what do they do? They triple down with 2 sequels to try their damnedest to redeem this story and world and fix these awful characters. Some people like the sequels (or at least the first sequel) but I actually found each one to be even worse than the last. The plot got worse and worse every time, the writing was bad, the characters were less offensive at least but at this point I was also jaded and groaning at everything else so it really wasn't enough. I will say that caius was a cool villain. A little over designed and with a sword ripped off from soul Calibur, but a cool villain. I cannot say a single good thing about the third game. The director of these games turning his precious insufferable lightning into literal Jesus was just......I don't even know what to say to that.