Gameplay
Tales of the Abyss is a 3D role-playing game with many of the standard features of these games: a game world made up of locations connected by a traversable overworld, a full party of playable characters, and a complex battle system. In addition, it has several features unique to Tales games, such as skits, cooking, titles, and grade.
World
The world of Tales of the Abyss is made of towns and dungeons connected via an overworld map. In towns, you can find all the amenities you might need: inns to sleep in to restore health, item vendors selling weapons, armor, and items, and grocers that sell ingredients to use for cooking.
The game's economy is particularly interesting: unlike many RPGs, where an item will usually have a set sale price regardless of when and where you buy it, here prices fluctuate based on where you are and what's happening in the world around you. For example, Engeve, one of the first places you visit, is a farming village and thus sells lots of different cooking ingredients, and when you first arrive you can stock up pretty cheaply. Soon after, though, there's an incident nearby, and if you go back you'll find that it's caused prices to rise sharply.
While towns are generally safe, with plenty of NPCs selling items, chatting, and giving out sidequests, once you leave you're in a standard RPG overworld, filled with monsters to fight and hidden items to find. Like most overworlds, you're first confined to traveling on foot, but you can eventually access different vehicles to make moving around easier. Your real goal, though, is the dungeons, which again are fairly standard for the genre: monsters, item chests, and the occasional secret passageway or optional side area. At the end, you'll usually find a boss fight, and then it's off to the next location.
Battles
The Tales series uses a real-time battle system, where you and your enemies are all placed in a battlefield and get to whale on each other more or less as you wish. As the player, you may have up to four of your party members in battle at a time, and can choose which one to actively control. The rest are computer-controlled, but you can customize their behaviour, such as which enemies to target first, how often to use artes, and when to heal or defend.
Each character uses a different type of weapon and has a different playstyle, such as Luke's slower but harder-hitting sword attacks, or Tear's focus on status effects and area-of-effect healing over dealing direct damage. Characters, and enemies, all also have a unique selection of artes, which can be either physical attacks (which take effect immediately) or spells (which require casting time and can be interrupted by enemy attacks). Both types of artes consume TP, the game's equivalent of magic points.
A complicating feature is the Field of Fonons system: some artes will create a temporary 'field' on the battlefield charged with a certain element, and using a compatible arte while standing in this field will transform it into a new arte with that same element, which can open up additional strategies.
Finally, each playable character has a Mystic Arte, a unique special attack that does immense damage to any enemy it hits, and which can be used by filling a special Overlimit bar. Some boss enemies also have Mystic Artes, though, so watch out...
Outside of battle, characters can be customized both with traditional equipment such as swords and armor, and by equipping Capacity Cores, which give stat boosts and can help with learning AD skills for passive benefits in battle.
Sidequests
Like any large-scale RPG, there are plenty of optional sidequests you can take to get various rewards, from special items to plot or worldbuilding information. Some of these are easy to find and complete: when you enter a town, just talk to everyone there, and someone will likely ask you to run some sort of errand, like bringing them a particular item, in exchange for a small reward. Some are more difficult, such as optional super-hard boss challenges that reward you with rare and powerful equipment. And some are truly fiendish.
The Tales series games of this era are somewhat notorious for having extended sidequests that provide substantial rewards, whether that's great items or complex and interesting subplots, but are extremely difficult to find, let alone complete. Tales of the Abyss is no exception, and is possibly the worst offender, with one particular subplot requiring revisiting several optional locations at very specific points in the story to successfully finish. The 3DS rerelease made the requirements for some of these sidequests less strict, but completing every single one is still going to involve referencing a guide. Luckily, none of these sidequests are required to understand and enjoy the game! They simply add additional depth to those who really want to dig deep into the world and are open to a challenge.
Unique features
Tales of the Abyss also has several systems that are unique to the series, such as cooking, skits, titles, and grade.
Cooking
Cooking is a way to obtain temporary effects for your party; cooking different meals can heal HP, cure status effects, or give temporary stat boosts. To cook, you must first obtain recipes, usually from books or NPCs, and buy the necessary ingredients. You can also choose which character should make the recipe, which can affect the final result: once a character has successfully cooked a particular meal enough times, they might unlock a unique variation that uses additional ingredients to get a bonus to its effect. Successfully cooking also raises a character's cooking skill, which is good, because some characters start off... not very good at cooking.
Skits
Skits are almost entirely a storytelling device. By performing certain actions, such as visiting certain locations, sleeping in inns, or just advancing the story to a certain point, you will receive a prompt to view an associated skit. In it, you get to watch your party have conversations about where they are, what's happening, how they're feeling, or how they've been spending their time between story beats. Most skits don't have a practical gameplay purpose, but they're great for learning more about the characters and the world. Unfortunately, though, while skits were voiced in Japanese, they weren't dubbed for the English release, which leaves them awkwardly silent.
Titles
Titles are an optional feature with no directly beneficial effects; they function as a sort of achievement system. Each character starts with a title that is descriptive of some aspect of their personality, status, or life: Luke, for example, starts with the title "Duke's Son." Additional titles can be obtained in several ways, such as playing through the game's plot, completing sidequests, or filling out collections of information like item lists or bestiaries. Most titles have no effects, but there are a few which change the character's costume. Some alternate costumes look cool, while some... don't. Still, for the video game completionist, collecting as many titles as possible adds an additional layer of challenge.
Grade
Grade is the last major unique feature in the Tales series, and functions as both a sort of scoring system and a special currency. Every battle awards grade based on how well you performed, with more difficult battles, such as boss battles, giving you the chance to earn larger amounts of grade. (Alternately, if you do a really bad job, it's possible to get a negative amount of grade, reducing your overall total.) Grade isn't valuable except as a performance indicator during the game itself, but if you manage to make it to the end and decide to start another playthrough, you can use your accumulated grade in the Grade Shop to buy benefits which will be applied to your New Game+. These benefits can range from carrying over data or items to changing how much experience your characters earn.