Mechanic Changes:


Ice types get a 50% defense boost in Hail.
Normal is supereffective against Fairy.
Poison is supereffective against Water.
Ice resists Dragon and Flying.
Grass resists Rock, and is no longer weak to Ice.
Bugs are no longer resisted by Steel, Fairy and Ghost.
Rock types absorb SR on switch-in while Ground types absorb Spikes on switch-in.
Steel resists Dark again (but not Ghost) .
Water types are immune to burn, Fire types are immune to freeze, Grass types are immune to sleep, and Ground types are immune to paralysis.


Ability Changes:


Air Lock removes weather completely apon switching in.
Arena Trap and Shadow Tag are nulliffied if the opponent has higher speed and/or after the user is hit.
Aura Break, as well as reversing auras, reverses user-affecting abilities (Huge Power, Hustle), not opponent affecting abilities (Intimidate, Forewarn)
Battle Armor and Shell Armor reduce damage by 12.5%, while Magma Armor has the same effect but is also immune to Sandstorm and Hail, and if a pokemon with these abilities and uses Shell Smash, their ability will be removed and they will get an extra +1 speed.
Defeatist lowers attack and special attack by 2 apon having 50% less HP. If the user heals and has over 50% HP, the stat drops will be removed.
Flash Fire raises Special Attack by 1 when hit by a Fire type attack.
Forecast makes Weather moves last 8 turns normally (but Weather Rocks do not stack up), as well as the normal effect on Castform.
Forewarn reveals 2/4 of any of the foe's moves.
Friend Guard reduces damage the more happy the other Pokemon is. (255 = 15%, 200 = 12.5%, 150 = 10%, 100 = 7.5%, 50 = 5%, 0 = 2.5%)
Grass Pelt raises Defense by 1 when hit by a Grass-type attack. Immunity to Grass.
Healer has a 30% chance of removing any Pokemon's status in your party.
Infiltrator ignores all Protect, Guard and Shield variants, along with Substitute, Safeguard etc.
Heavy Metal doubles weight, and decreases speed by 50%.
Insomnia raises Speed by 1 when hit by a Sleep-Inducing Attack. (Grass types with Insomnia are unaffected by this ability)
Iron Fist gives moves affected 50% more damage, instead of 20%.
Justified makes the Pokemon immune to Dark type attacks as well as raising attack.
Light Metal halves weight, and raises speed by 50%.
Magician swaps the user's item with the opponent's apon entering battle (this only happens once) .
Magic Guard prevents hitting yourself in confusion and recoil.
Mega Launcher also raises the power of blast and beam moves; Solar Beam, Hyper Beam, Ice Beam, Focus Blast and Bubblebeam.
Mold Breaker also bypasses natural immunities.
Parental Bond deals 80% damage, then 40% damage instead of 100% then 50%.
Pickup removes all entry hazards on switch-in, after taking damage.
Pure Power doubles a Pokemon's Special Attack stat.
Sand Veil reduces accuracy of both pokemon on the field by 10%, and only lasts if the user is still on the field.
Slow Start halves Attack and Special Attack for five turns.
Snow Cloak doubles the Pokemon's Speed in Hail.
Stench deals damage to a opponent (with no status condition) every turn (same as burn, but with no status drop, with Poison and Steel types unaffected)
Symbiosis nullifies the opponent's item as soon as the users battle.
Telepathy negates negative effects of moves used by the user, such as recoil and negative stat multipliers.
Victory Star raises move accuracy to the user by 40%.
Zen Mode changes the Pokemon to Zen Mode as soon as it enters the battle.


Move Changes:


Boomburst, Hyper Voice, Round, Uproar, Relic Song, Snore, Force Palm and Vacuum Wave can hit Ghost types.
Bone Rush, Bonemerang, Bone Club, Horn Drill and Mud Shot can hit flying types.
Night Shade, Shadow Sneak, Shadow Force and Phantom Force can hit Normal Types. Electroweb can hit Ground types.
Dragonbreath can hit Fairy types.
Acid and Acid Spray deal 60 (has 30% chance to lower Sp.Def) and 70 (has 10% chance to lower Sp.Def) damage respectively, and are supereffective against Steel.

Snore deals 70 base damage, is either Physical or Special depending on the user's highest attack stat, and deals double damage when asleep.
Flower Shield acts like Protect; and raises defence and special defence by 1 if the opponent attacked that turn.
Rock Smash lowers the opponent's defence by one 100% of the time.
Cut lowers the opponent's speed by one 100% of the time and is grass-type.
Twister uses Sp.Attack and Defense, and deals 80 damage.
Feint is Dark type, physical and deals 40 damage normally and double if the user is using Protect/King's Shield/etc.
Mud Sport weakens the power of physical moves for 5 turns while Water Sport weakens the power of special moves for 5 turns. They are affected by Light Clay.
Quash harshly lowers an opponent's speed.
Hyperspace Hole deals 110 damage instead of 100, ignores immunities, lasts one turn, and is Ghost type.
Diamond Storm deals 110 damage instead of 100, as well as it's intended effect.
Thousand Arrows and Thousand Waves are learned naturally by Zygarde, and do 95 damage each.
Barrage is Grass-type, and deals 30 (x2-5) damage.
Hold Back and False Swipe deal 50 each, and double if the opponent has full health. Hold Back is also special.
Hold Hands raises Special Defence by 1.
A Pokemon affected by Powder will lose 25% HP if it uses a Super Effective move.
Crafty Shield blocks EVERY status move used against the user.
Both Dual Chop and Twineedle deal 45 (x2) damage.
Mega Punch deals 120 damage, but has 75% accuracy, and is Fighting type.
Mega Kick is Fighting type.
Triple Kick deals 40 damage (x1-3) but has 80% accuracy.
Octazooka deals 100 damage, and has 90% accuracy.
Muddy Water has 95% accuracy.
Defend Order raises Defense and Sp.Defense by 2.
Glaciate deals 95 damage, as well as it's normal effects.
Mat Block deals 1/6 damage to a Pokemon that attacked the Block, during that turn.
Shadow Punch deals 75 damage, 100% accuracy and a 20% chance for a flinch.
Present is special, ice type, and deals from 40-160 damage (80: 40%, 120: 30%, 40: 20%, 160: 10%)
Relic Song deals 130 damage, bypasses immunities and is either physical or special depending on the user's highest attack stat.
Quiver Dance raises Speed by 2, but Special Attack and Special Def by 1.
OHKO Moves have changed; they deal 150 damage each, and act like Giga Impact. Guillotine is Steel type while Horn Drill is Ground type.
Nuzzle deals 75 damage, and if physical or special depending on the user's highest attack stat (not base). Default is Physical.
Fairy Wind, Ember, Vine Whip, Thundershock, Gust, Peck, Smog, Confusion, Mud Shot, Rock Throw, Feint Attack, Dragon Rage, Struggle Bug and Silver Wind have +1 priority. They all deal 40 damage now, 100 accuracy and have no other effects.
Leech Seed, Rock Slide, Night Daze, Dual Chop, Bone Rush, Bonemerang and Drill Run have 100% accuracy.
Fire Pledge has a 40% burn chance, Grass Pledge has a 40% sleep chance while Water Pledge has a 40% poison chance.
Ancient Power/Omnious Wind deal 75 damage, as well as their normal effects.

Bug Bite is boosted by Strong Jaw, and deals 70 damage along with it's nornal affect.
Hyper Fang and Super Fang are boosted by Strong Jaw too (Super Fang removing 75% health).
Similarly, Sucker Punch is boosted by Iron Fist.

Stealth Rock deals...
15% damage at 4x effectiveness
10% damage at 2x effectiveness
5% damage normally
2.5% damage at 1/2 resistance
1.25% damage at 1/4 resistance


Pokemon Changes:


Luvdisc has 65 Defense as well as 65 Special Defense.
Spinarak has 60 Speed while Ariados has 90.
Ledyba has these stats:
40-55-25-40-40-30-80
Ledian has these stats:
55-85-55-50-55-110
Dustox has 100 base Sp.Defense, and has it's HP and Speed stats swapped.
Nidoran male has 42 Special Attack.
Vulpix has these stats: 60-45-70-50-70-55
Ninetales has these stats:
95-80-100-90-110-90
Meowth has base 70 Attack, 80 Speed,  35 Special Defense and 45 Special Attack.
Persian has base 95 Attack, 105 Speed, 70 Special Attack and 60 Special Defense.
Cubchoo has 65 HP, 80 Attack and 60 Special Attack.
Beartic has 115 Attack and 80 Special Attack.
Swalot has these stats:
75-70-120-70-105-60
Gulpin has these stats:
55-40-70-40-55-40
Meloetta Pirouette can be used whenever.
Swablu has these stats:
45-40-60-65-75-50
Altaria has these stats:
75-70-90-95-105-80
Normal Castform has 80 for each stat. (Total 480)
Hail Castform has:
80-85-60-85-60-110
Sun Castform has:
80-95-60-95-60-90
Rain Castform has:
80-60-95-60-95-80
Farfetch'd and Delibird both have 52 HP and 90 Speed. Delibird now also has 58 Attack, 62 Defense and 55 Sp.Defense.
Igglybuff has these stats:
45-40-55-25-23-20
Jigglypuff has these stats:
85-55-65-45-43-30
Wigglytuff has these stats:
110-85-90-70-68-60
Mega Alakazam has 105 Special Defense.


Item Changes:


The Drives affect Techno Blast more; Ice TB has 20% Freeze, Fire TB has 20% Burn, Water TB has 20% Poison and Electric TB has 20% Paralysis.
A Focus Band raises accuracy by 50%, but acts like a Choice item.
Honey raises speed to +1 for the user, and -1 if the opponent hits a Pokemon holding honey.
All types of Gems are usable, including the Fairy Gem.
The Amulet Coin acts like all Pokemon on the field have Serene Grace, but does not add on to serene grace's multiplier.
The Cleanse Tag has a 20% chance of healing a status condition each turn.
The Destiny Knot acts like the ability synchronise.
The Eject Button and Red Card work even if the Pokemon faints.
The Electrizer paralyzes a Pokemon if they make contact with the Pokemon holding it once.
The Magmarizer burns a Pokemon if they make contact with the Pokemon holding it once.
Light Clay extends Safeguard, Wonder Room, Trick Room, Mud Sport etc. as well as Reflect and Light Screen.
The Mental Herb reduces 4x effective damage to 2x but 1/4 resistance is lowered to 1/2 as well.
Metal Powder boosts any Pokemon's defence and special defence by 25% but lowers attack and defence by 10%.
Bright Powder boosts any Pokemon's attack and special attack by 25% but lowers defence and special defence by 10%.
Quick Powder boosts any Pokemon's speed by 30% but lowers HP by 10% every turn.
The Muscle Band and Wise Glasses removes negative effects of Physical/Special moves respectively in exchange for half speed.
The Satchet and Whipped Dream both act like Pixie Plate.
The Protector reduces damage by 12.5%, and does not add on to Friend Guard/Shell Armor/Battle Armor.
The Pure Insence removes the holder's second type. (E.g  Gengar will be pure Ghost while shedinja will be pure bug)
The Reaper Cloth raises Defense by 50%, but the holder can not use status moves.
The Scope Lens boost the critical ratio by 20%.
The Wide Lens boost accuracy by 20%, but does not add on to Victory Star.
The Zoom Lens boost accuracy by 30% if the holder moves after the opponent, but does not add on to Victory Star.
The Smoke Ball allows the user to escape Shadow Tag and Arena Trap or any other trapping moves.
The Soothe Bell lowers recoil by 33.3%.
The Red, Blue, Pink, Green and Yellow Scarfs increase Attack, Defence, Sp.Attack, Sp.Defense and Speed by 1.1x.
The Lucky Punch has a chance to deal 20% recoil when the user is hit.
The Sweet Heart triples Luvdisc's Defense and Sp.Defense.
Old Gateau heals any status condition and heals 20% HP when consumed.
The Grip Claw acts like Shadow Tag/Arena Trap but the user cannot use status moves.
The Pretty Wing acts like the ability Cute Charm.
Shoal Shell protects the user from all entry hazards, including Leech Seed.
Shoal Salt stops recharge (from moves like Fissure AND abilities like Truant) one time use only.
The Upgrade swaps the user's Attack with Special Attack, but is a choice item. The Dubious Disc swaps the user's Defense with Special Defense, but is a choice item.
Soul Dew swaps the user's lowest stat with it's highest but is a choice item (HP is ignored).
The Dragon Scale prevents moves that restrict 'move-restricting' moves (E.g. Imprison, Taunt etc.).


Other:


All Hidden Abilities and Diancie/Volcanion/Hoopa/AZ's Floette/Mega Latias/Mega Latios are released. (Mega Legendaries in AlphaUbers while the rest are in AlphaOU)
Certain Pokemon are classed as 'NL' (Natural Levitaters) which means they are immune to Ground type moves whatever their ability. (E.g. Magnezone, Metang, Klinklang, Haunter etc.)

Pokemon listed here are now AFFECTED by Ground-type moves:

Pidgey
Spearow
Gengar
Farfetch'd
Doduo
Dodrio
Hoothoot
Natu
Murkrow
Delibird
Tailow
Starly
Chatot
Shaymin-S
Pidove
Archen
Ducklett
Rufflet
Vullaby
Fletchling
Hawlucha