The worst reason to hate Final Fantasy XIII (FFXIII) is because of its linearity. Non-linearity doesn't necessarily improve a game, and following a constrained path doesn't necessarily make it worse. All Final Fantasy games, including the most highly praised ones, have been essentially linear in both story and world design, and FFXIII is not even unique in degree, given Final Fantasy X (FFX). A player's desire to break out of a constrained experience is usually not a result of linearity per se, but instead reflects a failure of the game's story to engage that player. "Failure" is a reasonably good description of FFXIII's story, which is surprising because Square-Enix had done so much of this stuff before.
Final Fantasy XIII concerns the plight of a planetoid called Cocoon that floats above a world called Gran Pulse. This "floating continent" motif should be familiar from Final Fantasy III, VI, and the more recent Revenant Wings. Cocoon is inhabited by millions of people and run by powerful, but limited beings called fal'Cie. The game's characters are turned into l'Cie, thralls of the fal'Cie who must complete a particular task and suffer the horrible fate of turning to crystal, or, should they fail or take too long, suffer the even more horrible fate of turning into a monster. The party's task is to transform into an unstoppable incarnation of rage called Ragnarok, and kill a fal'Cie called Orphan, who supplies all the energy that keeps Cocoon floating. For reasons that are too tedious to relate, both Cocoon and Pulse fal'Cie want this to happen, but fight the party at every turn anyway.
The story's terminology is unbearable and endlessly repeated, but the more serious flaw in this setup is that it contains no problems that the game's mechanics are equipped to solve. The fundamental problem of FFX, in contrast, matched the mechanics perfectly: the goal was to kill Sin, and here we had a party that was already pretty effective at killing things. The logic of the quest was evident from the beginning. Here, the goal was to not kill something, even though killing seems to be the only thing the party is competent at. The gameplay and larger story are completely at odds with one another.
The game never resolves this dissonance, but instead carries it into the final battle. Having traversed all of Cocoon and much of Gran Pulse in an effort to avoid killing Orphan, the party goes to Cocoon and... kills Orphan. Then, in a moment of deus ex machina the player has been given no reason to suspect is possible, Vanille and Fang transform into Ragnarok, and somehow repurpose the killing machine into a giant glove to catch Cocoon in its fall. Then all the other characters lose their l'Cie brands, turn back from crystal, and live happily ever after.
Pat, happy endings are hardly unusual in RPGs, but FFXIII's is especially untrue to the preceding story. From very early on, the game emphasizes that l'Cie cannot avoid a horrible fate. Success and failure at the appointed task both result in a horrifying living death. There's no happy ending to be had if you buy into that premise, so the developers simply sprung one that betrayed both the world they had built and the story they had told.
Once upon a time, the folks at Square had the guts to stay true to their stories. The logic of FFX's story didn't allow a happy ending, so it didn't have one, and was all the better for it. An unearned happy ending is not better than a logical, sad one.
When the larger plot isn't satisfactory, one hopes to at least enjoy the characters of an RPG, but this proved largely impossible. Nearly every member of FFXIII's cast was positively insufferable. Sazh was the exception, but I've seen better Danny Glover impersonations. While there are perfectly good reasons to hate everybody else in the cast (especially Vanille), my personal ire was for Snow. For reasons I have outlined before, I don't care for the noble idiot character, and Snow is a particularly egregious example — an enormously strong frat boy with the mentality of a five year old playing cops and robbers.
Of course, character development can't really happen without character flaws, but FFXIII has very little to see in this regard. Fang, Snow, and Sazh are completely static. Vanille doesn't really change over the course of the game either, although she had apparently changed since becoming crystal the first time. Lightning has an interesting arc where she tries to redeem herself for failing Serah by saving Hope, but this plot is dropped entirely almost as soon as it crystallizes, about a third of the way through the game.
That leaves Tidus, who hates a father figure he blames for his mother's death, but after a climactic confrontation, reconciles with the man and tries to make him proud. Or rather, I mean Hope, who shares essentially the same character arc. In Hope's case, however, the father figure is not his literal father, but instead is Snow, the character I found most irritating. Hope's transition from wimpy whiner to party cheerleader is plausible, if you remember how teenagers can be, but it treads no new ground, and involves him changing from a character I dislike into a character I dislike even more.
The only rewarding part of the storytelling is the relationship between the story and the local world structure. Simon Ferrari has articulated the view that the linear portions of the game represent the constrained existence of Cocoon while the open portions represent the wild freedom of Gran Pulse. As such, the interplay between the game's two main settings is encoded in the structure of its spaces.
I don't entirely agree, because Gran Pulse is also a series of tubes. Only the Archylte Steppe is open; tubes lead off from it in all directions into ruined cities, canyons, grottos, and mines. Thus, the freedom of the Archylte Steppe is atypical, even for Gran Pulse. This suggests that the design is not about showing a contrast between Cocoon and Pulse so much as revealing the party's state of mind. Here, for a moment, the characters lack a clear sense of purpose or idea of where to go. The vast, open Steppe reflects this indecision. Once the characters have a destination again, the game closes back down into tubes, carrying the party inexorably towards their fate once more.
Of course, this synergy is not a triumph of FFXIII's own design; like so much of the rest, it has been copied. The brief moment of openness between constrained paths, corresponding to a feeling of uncertainty, should be familiar to anyone who reached the Calm Lands in FFX.
There is little to admire in the fiction of Final Fantasy XIII and of those few things nearly all were drawn in part or whole from better, preceding entries in the series. FFXIII is an unsatisfying story about tedious people built on a thinly-conceived and largely boring mythology using worn-out tropes. The game is devoid of originality, compelling characters, or a plot that meaningfully relates to what the player is doing or even the virtual spaces in which he does it. It wasn't the tubes that made FFXIII bad, it was what was in them.