Final Fantasy XIII is an odd beast of a game. FF XIII is a game that blurs the lines between what we expect from Japanese and Western game design. It moves away from what has become staple for the series resulting in an odd mixture of action and rpg elements.
Paradigm shifts are the player's means of switching the class makeup of your party in the midst of combat. Each character has their own set of roles they specialize in. There's your basic Tank (Sentinel), Healer (Medic), and Damage dealer (Ravager or Commando). To add to that, there are the Synergist and Saboteur classes that focus more on inflicting status ailments or buffing up your party. The player has a set number of slots where you can mix and match your classes. You can have a paradigm of three Ravagers for quick chain combos, or have a Sentinel and two Healers if you need to recover. The key to success is fully utilizing all of these classes through Paradigm Shifts. You have to switch paradigms to get your allies to use the desired skills for the problem at hand. Need a heal? Switch to a paradigm with a healer and they'll cast cure. The AI is smart enough to depend upon in these situations. Switching on the fly constantly during each combat scenario and making a variety of different class compositions is crucial to your success. Different enemy types demand different strategies.
The Japanese names for the classes and Paradigm shifts are much better than the NA ones.
The main issue with all of this is its emphasis on trial and error. Death is pretty much a constant with each new creature set the player is faced with. Since the game checkpoints before every battle and offers you a retry feature, they expect you to die a lot before you find the right strategy to use. I found my self dieing constantly once I got pretty deep into the game. This also made the bulk of FFXIII pretty exhausting. The fights are so fast and frantic that there really isn't any room for breaks. I was forced to pause the game for a couple minutes after nearly every battle.
Another odd thing to note about the combat is how slowly it takes for everything to open up to the player. The first 20 hours of the game are almost like a tutorial. At first you're given a full party of characters but don't have control over the paradigms (Although that only happens for about 45 minutes).The rest of this consists of you playing in two-man parties in order to familiarize the player with the concept of mixing class types together. Finally, when you reach the 20 hour point you get a full party and the ability to pick and choose placements.
It isn't as bad as it sounds though. You may know you're not getting everything there's to offer but the game does a good job of giving the player challenge to make up for the fact. The experience is similar to when you're playing an MMO and you're aware that you haven't gotten the full extent of the class you're playing but you can still derive enjoyment from what you have been given at that time. It's a teaching device that can still be fun and not pandering.
Despite the combat system's glaring flaws, FF XIII has hit a sweet spot between forcing the player to stay on their toes and still being methodical about your approach to a battle. There's fun to be had in mixing and matching classes to see how they will stand against foes and then trying to pull them off in combat. The combat was hectic but it still demanded that the player step back and look at the bigger picture to develop an effective strategy. Everything is quick and FF XIII demands that the player keep up.
Most of FF XIII is pretty quick, oddly enough. Its moves at a pace that previous games of the series aren't necessarily known for. The game starts off by throwing the player into combat as soon as the first cutscene is done. The story is also lacking the set up we're used to at the start of these kind of games (This is later resolved through flashbacks). The levels also take a number from action games like Modern Warfare or Gears of War, putting priority on the road ahead and being very straightforward. Literally, the levels are designed in straight paths that occasionally branch off only to lead the player to items. Everything tends to open up around the 20-hour mark but the level structure still relies heavily on its old habits despite that. It cuts out the fat and boils FF XIII down to its essentials: Combat and story. It alleviates distraction and forces the player to focus on the path ahead. In its attempt to allow the player to see everything it has to offer, the useless content that would detract from that was cut out. Taking away things like substantial side-quests and putting more focus on the main story.
Side activities are still present but most of them are basic kill quests. They'll help the player in the long run but it doesn't demand that the you do them to derive more enjoyment from the main game. It doesn't want the attention to be taken away from the story so it doesn't inflict the nagging suspicion that the player is missing out on any important content. FF XIII still had a clear idea on what is important and where main objective lied. Everything is focussed and time never feels wasted.