I understand this definition very much hinges upon which aspects of the game are "essential" for play and which ones aren't, but I don't think anyone can deny that changing which party members you're using is essential gameplay in a Final Fantasy game. That you're restricted from it until the second half of the game starts (and not just for plot reasons) is indefensible.
Trying to define it as the point where the central conflict is revealed and the central plot takes its direction in full force (like the end of Ostagar in DAO) also looks bad in Final Fantasy's favor because the scene with Barthandelus is the first point in the game where you find out what you're actually supposed to be doing (well, more like given vague hints of what you're supposed to be doing, which the game never actually explains even at the end: FF13's strong point is most certainly not its clarity and focus). Which is the same point where you finally get to control which party members you're using, and (IIRC) the point where you can finally upgrade any paradigms you want and not just the 2-3 for each character the designers picked for you.
The linear portions of FF13 are defined by pointless wandering. Everything the characters do leads to absolutely nowhere until Sazh and Vanille get captured (which could have happened at any time due to Creepy Plot Device Kid and didn't need to wait until Nautilus except the developers spent money on the setpieces there and wanted you to look at them). The only ones trying to actually do anything are Lightning and Hope trying to take down Eden, but this gets abandoned halfway through for seemingly no reason.By the way, Gran Pulse sucked for me. So much pointless wandering and random battles, with no plot or direction. I like FF13 more when it's linear, at least it does that well.
Then once you're done wandering around in the fields of Gran Pulse, the party decides to go to Oerba to get rid of their l'Cie brands (exactly how going to Oerba will help accomplish this is never explained) which also leads to absolutely nothing. Oerba, the entire goal of 20 hours of gameplay, was a complete dead end. The only reason the plot doesn't just stop right there is because Barthandelus just so happens to swoop down and take them back to Cocoon when they get there.
And then... the ending. Do I even need to explain the problems with the ending?