Almost every feature of the original PlayStation 2 version of Final Fantasy XII has been completely restored.
At the very bottom of this list are a few features of The Zodiac Age that have been left alone and have not been changed.

A lot of thorough research and testing was done to make sure everything was as accurate as possible, including gathering data directly from the PS2 game CD (thank you, ffgriever!). Because there were so many changes, I may have forgotten to list something. If you are wondering about something specific, feel free to ask!

You can also check out this Final Fantasy wiki article if you want to see a more detailed list of differences between The Zodiac Age (International Zodiac Job System) and the PS2 Original. Note that some information in the wiki may not always be 100% accurate.

Orange text = Feature of Final Fantasy XII The Zodiac Age
Blue text = Feature of Final Fantasy XII (PlayStation 2 Original)

System

• The Original License Board has been restored and replaces The Zodiac Job System.
• Licenses added in The Zodiac Age are not available.
• Licenses have their original PS2 names, contents, LP costs, etc.
• Trial Mode is not available.
• A remastered version of the PS2 title screen cinematic can be played in-game. (Credits to the Square Enix YouTube channel) (optional)
  

• The contents of all unlearned licenses on the license board display a "???" until an adjacent license is learned instead of the contents always being visible. (w/ The Insurgent's Past) (can be disabled)
• If preferred, The Classic Effect Capacity system can be used (disabled by default), which uses the Original PS2 version's 8 effect values instead of The Zodiac Age's 24 effect values. (w/ The Insurgent's Past) (optional)
  

• Playable character's starting LP, gambits, equipment, licenses, and base stats have been restored to their original PS2 values.
  

• Guests cannot be controlled by the player. (w/ The Insurgent's Past) (can be disabled)
• Guest's gambits cannot be changed or turned off. (w/ The Insurgent's Past) (can be disabled)
• Items used by guests do not expend the player's inventory. (w/ The Insurgent's Past) (can be disabled)
  

• Guests do not gain EXP and level up. (w/ The Insurgent's Past) (can be disabled)
• Guests do not give the player items and gil when joining the party.
• All guest's weapon stats, gambits, abilities, equipment, augments, etc. have been restored to their original PS2 values.
  

• All Magicks and Technicks can be purchased in shops and are no longer found in chests.
• All Magicks and Technicks have their original PS2 stats, accuracy, power, effectiveness, etc. (w/ The Insurgent's Past)
  

• Magicks and Technicks have their original PS2 categories.
• Spells have a charge time of 30 instead of 23.
• Aqua, Aquara, and Aquaga have been renamed Water, Watera, and Waterga.
• Stamp can target allies.
• Since Magicks now have their original PS2 categories, Shades of black can cast Ardor and no longer casts Blind, Blindga, Silence, Silencega, Sleep, Sleepga, Poison, and Toxify.
  

• The sixteen new gambits added to The Zodiac Age are not available.
• Gambits can be found in treasure chests.
  

• Mist abilities deplete MP instead of using up a seperate Mist value.  (w/ The Insurgent's Past) (can be disabled)
• A new MP Gauge is added to a character when obtaining a new Esper or quickening. Each character can obtain up to 3 MP gauges, called "Mist Charges", with each gauge possessing the same maximum MP limit as the character's original max MP. (w/ The Insurgent's Past) (can be disabled)
• The UI for the Mist Bar can be displayed as a Mist Charge/ MP Gauge. (can be disabled)
• Nordswain's Glow is renamed Northswain's Glow.
• Certain Concurrences require more quickenings to activate.
  

• Espers cannot be controlled by the player. (w/ The Insurgent's Past) (can be disabled)
• Esper's gambits cannot be turned off. (w/ The Insurgent's Past) (can be disabled)
• Espers do not require MP to cast magicks. (w/ The Insurgent's Past) (can be disabled)
• Esper's stats, magicks, equipment, gambits, etc. have been restored to their original PS2 values.
• Esper's technicks have their original PS2 stats, power, formulas, & charge times.
• Summon durations are now one minute and thirty seconds instead of four minutes and ten seconds.
• Esper summoning animations have been restored. (can be disabled)
• Zalera absorbs holy.
  

• All enemies and bosses in the game have their original PS2 stats, augments, abilities, steals, drops, etc.
• There is a 9999 damage limit. (w/ The Insurgent's Past) (can be disabled)
• The Safety augment nullifies Numerology, Wither, and Addle. (w/ The Insurgent's Past)
• Status effects have their original PS2 durations.
  

• Shops have their original PS2 inventory and update their inventory based on their original PS2 story progression requirements.
• Dark Matter can be acquired from the bazaar.
• Black Orbs in the Pharos at Ridorana will only drop after defeating every 3rd enemy instead of every time an enemy is defeated. (can be disabled)
  

• Items added in The Zodiac Age are not available.
• Items have their original PS2 names, effects, and potency.
• Chronos tears do not remove slow.
• Bacchus's Wine has a 50% chance of inflicting berserk instead of 100%.
  

• Equipment added in The Zodiac Age are not available.
• All Weapons, armors, and accessories have their original PS2 stats and effects.
• Spears, poles, rods, and staves cannot hit flying enemies.
• Mystic Armors do not all increase MP.
• The Gendarme is now the Escutcheon.
• Berserker Bracers are now the Armguard and reduce physical damage when HP critical.
• Gauntlets now provide an extra shield block augment instead of reducing physical damage when HP critical.
• The Turtleshell Choker now has it's original PS2 gil costs. (w/ The Insurgent's Past)
• Cat-ear Hood gives +20 vitality and +50 speed instead of turning LP gained into gil.
• Germinas Boots are now Jackboots and give the wearer immunity to immobilize instead of +20 vitality and +50 speed.
• Blackbelt gives the wearer immunity to disable and no longer gives the wearer immunity to immobilize.
• Equipping the Diamond Armlet gives the player a 90% chance of regular treasure and 10% chance of special treasure when opening chests instead of 95% and 5%. (w/ The Insurgent's Past)
  

• All sidequests and hunts have their original PS2 rewards.
• The Hunt Club Outfitters have their original PS2 inventory and rewards.
• The chop collecting quest in Archades requires nine chops to progress the story instead of three chops. (can be disabled)
  

• All treasure chests and traps have their original PS2 locations and contents.
• Treasure chests and traps added in The Zodiac Age are no longer available.
• Chests respawn after having visited 3 different unique areas instead of respawning immediately after zoning into an area. (w/ The Insurgent's Past) (can be disabled)
• The "forbidden chests" have been restored. Opening a forbidden chest will permanently prevent the Zodiac Spear chest from spawning in the Necrohol of Nabudis.
• The maps of the Ogir-Yensa Sandsea and Nam-Yensa Sandsea have been returned to their original PS2 locations.
• The Baknamy merchant in the Necrohol of Nabudis is located in the Hall of Effulgent Light instead of the Cloister of the Highborn.
  

• The Insurgent's Past added: Classic Treasure spawn mechanic (can be disabled)
• The Insurgent's Past added: Shops now work as they do in the original.
  

• The Insurgent's Past added: Unlearned license node contents will display "???" unless an adjacent license is learned. (can be disabled)
• The Insurgent's Past added: Classic Effect Capacity System (disabled by default) (can be enabled)
  

• Enemies have their Zodiac Age spawn mechanics. (e.g. Marilith spawns after 2 minutes instead of after 5 minutes.)
• The QoL features of the Zodiac Age can be used, which include 2x and 4x game speed, auto-saves, etc.