FFX was a very linear game in the sense that the story shuttled you along from area to area, without leaving a huge amount to explore. The reason why it wasn't a problem is because there are still a lot of extras to do in the games, and secrets to uncover. The story and writing were also great, save for two or three generally unlikeable characters.
People point at 13's flaws of being linear as just the map design. Sure, the map design is laughably linear with no places to go, but that wasn't the only problem. What people (and designers obviously) overlook is the importance of having things to do or discover within whatever map you have. (EDIT: That's the reason why FF13-2 and Lightning Returns still have a weird feeling of linearity, even though the maps are significantly bigger. There needs to be things to do and they need to be interesting). P4G has maps that are as linear as you can get but if "feels" less linear because the areas are jam packed with things to do and secrets to uncover. Not to mention the writing in P4G is absolutely stellar, and makes those secrets so interesting and addictive. 13 had absolutely nothing to uncover until the very end of the game, and when you finally had something else to do (side missions on pulse) it was as boring, bland, and poorly written as the rest of game.
What people are really missing in these games are secrets to uncover and non-essential side missions that are cleverly written or interesting in some way. Square seems to dump most of their money into graphics and QA (most of Square's games run flawlessly, which is admirable when compared to some other companies), but they don't realize that it's those little secrets which reward players for investing their time and attention into the world that makes players really appreciate it. TWEWY and KH are games that seem to understand that, and that's the biggest reason I still have some hope reserved for FF15, despite all the negativity generated from the 13 trilogy.