While Square Enix could be reasonably blamed for having a hand in the endless torrent of Video Game Sequels, the gameplay of the Final Fantasy titles varies wildly - for better or for worse. FF13 is one such game that made some bold changes in how it played; and the fan backlash was pretty huge. That said; you can call me a supporter.
Setting aside the plot of the game for now, the main complaint was Final Fantasy XIII being linear. Now, linearity in a game is incredibly common. From Super Mario Bros. to Call of Duty: Modern Warfare 3 - a non-diverging progression from beginning to end is the standard, and often to no detriment whatsoever. Even other Final Fantasy titles are in no way free-roaming.
What makes FF13's linearity so salient is that it displays this decision with bold colours. A mini-map is present at all times, condensing the (incredibly stunning) environments into a 'Head from A to B' schematic. After every plot point you encounter, the path behind you often tends to be sealed off - not that you have a reason to backtrack. Even the tried and tested formula of having towns filled with exposition-townsfolk and item shops have been distilled - buying items and upgrades can be done at the frequent save points.
There's a beauty in that. In a games industry where many Japanese RPGs are oft-filled with busywork and errands - forcing the player to go back to previously beaten areas; and many Western RPGs are devoted to sandbox exploration and hollow moral choices; FF13 is devoted to giving you a well told story and a great battle system with none of the fat.
With the extraneous gaming abstractions cut out, FF13 is free to focus on it's narrative - the planet of Cocoon is co-inhabited by humans and gargantuan aliens called the Pulse Fal'Cie. While these aliens are responsible for keeping the ecosystem in check, they often treat humans as pets - ruining lives and entire settlements in the process. The story flicks between Lightning - an ex-soldier on a personal mission to rescue her sister from becoming tainted by the Fal'Cie - and other citizens and soldiers who get dragged along for the ride - none of them have the same perspective, and as they traverse Cocoon (sometimes to explore, often to escape) we learn more about their individual motivations and the nature of the world they live in. It's gripping.
The story is broken up by inevitable battles with both monsters and military; using a system that stands out as the most simplified, yet demanding battle system used in a Final Fantasy game. While every party member has a dedicated role - mêlée attacks, healing, casting magic, etc. - you can 'Paradigm Shift' at any time to switch roles as the situation demands. The skill is not in being able to select attacks (you can let the game carry out actual attacking automatically); but in planning ahead enough to turn the tide of battle on a clever Shift. It takes very little time for battles that rely entirely on a good Shifting strategy to emerge unscathed. If you do happen to succumb to failure, the game will happily let you reload to just before the battle so you can try again. Even in death, FF13 is streamlined to you making progress.
A lot of gamers actively enjoy burning hours on repeatedly killing the same monsters to be strong enough to progress; some players are willing to accept a looser story for the promise of more immersion, and FF13 just simply isn't for those people. It perfectly caters to what I look for when gaming; a strong and compelling story with a battle system that challenges me, and none of the annoying busywork.
A sequel, Final Fantasy XIII-2 will hit stores in February, and in its press releases, it promises to put back the towns and meandering that its predecessor removed. A far more accessible decision, but for me, FF13 was a successful experiment in bringing the JRPG to the current generation of games.