It takes a lot of risks, and the end result is somewhat mixed.
The Final Fantasy series has experimented with its gameplay since the beginning, explaining why the quality from title to title has varied considerably. Final Fantasy XIII, the latest game in the series, sums up that spirit as perfectly as it can: certain parts come across as well written, highly involved fun, yet other moments are hackneyed portions that demonstrate what not to do in a game. However, in the end, Final Fantasy XIII is a game which improves on many of the concepts from previous games, yet still leaves quite a bit to be desired.
In fact, Final Fantasy XIII is a game that is incredibly focused on the story, even if that means neglecting other major aspects of the game. The environments are blatantly linear, featuring few diverging paths or complex puzzles. Linearity is not necessarily a bad thing, but the extent to which this game carries it makes it somewhat of a one-note game, with little replay value that cannot be exploited with a few saves. Everything that could divert the player's focus, like towns or side-quests, has either been stripped to its most basic level or removed altogether. This concept even extends to things indirectly related to the victims of this over-linearization, weapons being the most noteworthy. You can still buy them the traditional way, but because of the weapon upgrade system reminiscent of Final Fantasy VIII, this will never be necessary.
The game seems to know this, because over the course of the game, you are provided with more components for upgrading weapons than you are gil to buy newer weapons. Even if you find yourself unable to upgrade a weapon (no components, at the highest level, etc.), it is still better not to buy a new weapon, since the upgraded weapon will, more often than not, be vastly superior to any alternate weapons throughout the game. The only possible reason to purchase a new weapon over upgrading a current one is that new weapons can have new abilities attached to them, something the upgrade system noticeably lacks. There are a myriad of different items used to upgrade weapons, but the only difference between them seems to be how much experience they give for a weapon level up, missing a great opportunity to make the weapons about as upgradeable as the characters.
Like Final Fantasy X before it, characters in XIII gain experience points they can put towards abilities or attributes in something called a Crystarium. Battling is not completely necessary to gain points; occasionally (but usually after a major boss battle or change in perspective), the story will provide a bounty of crystarium points to spend almost immediately. Usually, this accompanies an expansion of the Crystarium, and usually, this expansion does not come soon enough. Final Fantasy XIII has a nasty habit of giving you too much experience, meaning you will regularly find yourself filling every portion of the Crystarium available to you at the time, letting whatever crystarium points you gain after it is full go to waste. Left with more points than you should have, the new level of the Crystarium, although larger than the one before it, will fill up just as quickly as its predecessor, making it a self-sustaining cycle. The game manages to fix this by the second disc, but until that point, the system feels less like the extensive customization of characters that it eventually becomes and more like checking off boxes until there are none left to check.
The same can be said of the battle mechanics: although it becomes an enjoyable system that requires both strategy and reflexes, you have to play through a mediocre glimpse of that system before you can utilize the full thing. The most notable flaw is that you cannot control characters other than yourself. It is not that the game is incapable of this, as the two upper shoulder buttons remain unused in battle, but rather that the game chooses not to do so. Of course, this decision brings a lot of superfluous problems with it, like that your options are limited to the skills of whatever character you happen to be controlling, and how the lead character's death will result in a game over. Other characters can be revived rather easily, but when one of them is in the lead, the battle immediately ends with their death. Running from deadly fights is not an option; escape requires death and starting over. Because several enemies and bosses (all of the summons, for example) can/will use instant death techniques, and because characters only dodge attacks through their commands, cheap deaths become commonplace in some (but far from all) areas. Avoiding death requires strategic use of the paradigm battle system.
(Most likely) to make up for the lack of direct character control, the paradigm system allows you to assign basic jobs to certain characters (medic for healing, sentinel to absorb enemy fire, synergist to buff party stats, etc.). Each job combination comprises one paradigm, of which you can use six in battle. However, because the game uses multiple perspectives constantly throughout the game, you will often find yourself creating new paradigms for new parties. Although the game provides basic paradigms and eventually allows customizable parties to alleviate the situation, it quickly becomes annoying to refill six slots with varied paradigms constantly. The six-slot limit and the fact that most abilities are limited to a single class may seem limiting, especially with the lack of control over multiple characters, but the system works surprisingly well, allowing just enough slots to fill all the positions you'll need in a given battle. The system may also seem like the largely non-interactive Gambit system of the previous game, but, learning from their mistakes, the interaction with paradigms is much greater than it was with gambits: even in random encounters, you will find yourself switching paradigms to the best effect, finishing battles quickly to get the most points possible (even if the score means nothing).
In fact, the battle system in general has improved greatly in Final Fantasy XIII. Active Time Battle, the main battle system of the series for eight games in the series, has almost become what it always should have been. Multiple bars fill up for multiple commands, allowing not only for variety in attack patterns, but also for attacks that take up multiple bars, adding a noticeable difference between spells of different levels and valid reasons to use low-level magic late in the game. It all adds a deep layer of strategy that the inability to target multiple enemies simultaneously takes away. Attacking multiple enemies requires either interrupting commands mid-slash to target another enemy, breaking flow and leaving you open to attack, or using wide-range attacks, which can be very limiting and takes longer than it should. Ultimately, the best strategy is to team up on one enemy, force them into stagger mode (wherein they take much higher damage and generally cannot fight back), inflict enough damage to defeat them, and repeat for any remaining enemies.
Aiding in this endeavor is the lack of enemy encounters; instead of wandering around a world map, looking for battles to appear at the whim of a random number generator (partially because this game no longer features a world map), enemies now appear in the environments just as they would in battle. Their attention is not always focused solely on you, the player; they sometimes fight other enemies, but it is always possible to sneak upon them (bosses excluded). While not perfect (enemies are bound to invisible walls, your allies can obliviously run through crowds and give away your position, etc.), it is still a highly satisfying system with a clear relation between effort and reward: you successfully catch an enemy from behind, are whisked away to an alternate battlefield (even though it is perfectly possible for the game to use the environments around you as battlefields themselves), and almost immediately place every enemy into the aforementioned stagger mode.
Bosses ( eidolons in particular), however, are almost the exact opposite: a frustrating nightmare only conquerable through the tried and true stratagem of trial and error. Most fights require an in-depth knowledge of the foe in order to defeat them, meaning every boss begins with a cast of the Libra spell to find out their weaknesses. What follows depends on the type of boss; usually, it is either "raise stats, stagger boss, attack relentlessly, repeat as necessary" for regular bosses, while eidolons require very specific and very different strategies.
The object of each summon fight is to fill up their meter before the lead character's death clock reaches zero. How the bar is filled depends on the summon, the character, and how far you are in battle. However, Final Fantasy XIII is somewhat elusive about how you fill the meter; as mentioned, Libra is necessary, but the requirements change over the course of the battle, but with no indication that they have. Of course, this will lead to a lot of deaths in the mandatory summon fights, moreso than in the regular gameplay. When you inevitably do beat the eidolon, the reward hardly matches all the effort it takes to earn it; each character has only one summon available at any time, and using it in battle usually wastes time for little damage, even against commonplace foes.
However, the summons do little to mar the image that Final Fantasy XIII has created for itself beforehand. Underneath the sci-fi/culturally medieval wrapping, it is still a Final Fantasy game with many (but not all) of the Final Fantasy mainstays. Although it relies heavily on the story (definitely too heavily for the tastes of some players), the gameplay is strong enough to stand on its own. In the end, though, it is a very linear game that takes risks with pre-established formulas, albeit with mixed results.