I made a rather long list on TV Tropes, with me adding more for Generation VIII, but here it is again. Bear in mind that not all of them are bad, per se, but are either incomplete (like Ripen), clash with the Pokémon that have them (like Tangling Hair), or are found mainly on Pokémon that need a buff (like Illuminate):
Keen Eye: Guarantees all damaging moves will not miss.
Forecast: Weather-setting moves receive +1 priority on top of changing Castform's appearance and type.
Innards Out: Deals a fixed damage equal to the user's total HP instead of HP remaining when knocked out, but only if it's still on Pyukumuku. Has its regular effect if the Ability is not on Pyukumuku.
Big Pecks: Defense and Special Defense cannot be lowered. Opponents' Attack stats cannot be raised. Flying-type moves get a 40% boost in damage.
Wonder Guard: Only super-effective moves can damage the user, but it now includes passive damage. That is, Shedinja can be hit by Sandstorm due to it originating as a Rock-type move, even if it's initiated with Sand Stream, but it cannot be hit by Leech Seed, Spikes, or Toxic.
Unnerve: Extends to the opponent being unable to use any items that are consumed (that is, used once and the item is no longer held), as well as Leftovers, rather than just Berries.
Analytic: The first time the Pokémon takes direct damage from a move, the move it uses on the next turn will have +1 priority added to it.
Bulletproof: Also protects against all pulse moves.
Comatose: Heals a small amount of HP at the end of each turn in addition to existing effects.
Queenly Majesty and Dazzling: If in a double battle, also protects allies from priority moves.
Electric Surge, Psychic Surge, Grassy Surge, and Misty Surge (NERF): Reduced from 5 turns to 3 turns of terrain. Terrain Extender restores it to 5 turns.
Water Compaction: Also grants immunity to Water-type moves unless the user is holding a Ring Target. When it's raining, this Pokémon takes half damage from physical attacks. If this Pokémon takes damage from an Ice-type move, damage is calculated as normal for that turn, but the damage multiplier for Ice-type attacks becomes not-very-effective from the next turn onward until this Pokémon faints or is switched out.
Illuminate: Light-based attacks (such as Flash Cannon and Dazzling Gleam) do double damage.
Zen Mode: Activates at the end of the first turn Darmanitan is in play instead of at 50% HP.
Defeatist: Activates at 25% remaining HP instead of at 50%.
Vital Spirit: This Pokémon is immune to Sleep. Any time an opponent's move that would normally induce Sleep is used on it, the Pokémon's Speed is increased 1 stage.
Corrosion: Poison-type attacks done to Steel-types and Poison-types are treated as if they're super-effective.
Strong Jaw: Biting moves' attack power is increased by 100% instead of 50% if the power is less than 80.
Grassy Pelt: Also increases Speed by 2 stages if on Grass or Grassy Terrain is in effect.
Receiver and Power of Alchemy: The Ability changes to that of the last Pokémon on its team that fainted, regardless of if it happened when the user was present or not.
Intimidate (NERF): In a double or triple battle, lowers the Attack of only the Pokémon directly in front of it and not adjacent ones.
Protean and Libero (SEMI-NERF): Activates at the beginning of the turn rather than right before the Pokémon attacks.
Rain Dish: Restores 1/8 of the Pokémon's maximum HP in rain instead of 1/16.
Slow Start: The 5 turns necessary to get the Pokémon's stats back to normal do not reset back to 5 when it switches out.
Stakeout: Also inflicts Paralysis on the replacement Pokémon if the opponent switches out except for Electric-types.
Tinted Lens: Not-very-effective moves get a x3 boost instead of x2 if they would ordinarily do half damage. If they would do quarter damage or less, the move gets a x4 boost. Moves with no effect still have no effect.
Water Veil: If the Pokémon is targeted by a Fire-type move, it will also have no effect and raises the Pokémon's Evasion by 1 stage.
Flower Gift: Raises two of the affected Pokémon's best stats (among Attack, Defense, Sp. Att, Sp. Def and Speed) rather than just Attack and Sp. Def. Cherrim's Sunshine Forme is also a Grass/Fire type.
Iron Fist: Increases damage multiplier to x1.5.
Light Metal: Steel-type moves gain +1 priority.
Heavy Metal: Physical Steel-type moves' attack power is increased by 50%.
Gluttony: Consumes the berry it holds the moment it can use its effect.
Long Reach: In addition to all moves done by this Pokémon considered contactless, if another Pokémon attacks with a contact move, that Pokémon takes damage equal to 25% of the damage dealt.
Tangling Hair and Gooey: Pokémon whose Speed is lowered by these Abilities are also no longer able to switch out unless they have Run Away or until the user faints or switches out.
Liquid Voice: Moves turned to Water-type by this Ability have their power increased by 20%.
Emergency Exit: In addition to its current effects, allows this Pokémon to switch in and attack on the same turn once per battle.
Shields Down: Changes Minior from Meteor Form to Core Form when at 50% or less HP but also fully restores Minior's HP. This Ability has no effect if it's on any Pokémon except Minior. Minior remains in Core Form for the rest of the battle. Using Shell Smash will immediately switch Minior from Meteor Form to Core Form regardless of how much HP it has but will not restore Minior's HP.
Berserk: Any time Special Attack rises due to this Ability, the next attack will have +1 priority.
Impostor: In addition to its current effect, if the opponent's level is equal to or greater than that of the user, the user's total HP changes to that of the opponent. If the user has already taken damage, the user's current HP will change such that the difference between its total HP and current HP remain the same. If this causes the user's HP to be 0 or less, the user will faint upon transformation. If the user is switched out, the total HP will revert to normal. If this causes the user's HP to be 0 or less, the user will faint if brought back out, before any transformation takes place.
Snow Cloak: The user gets the effects of Aurora Veil whenever Hail is active, though localized only to itself, instead of its current effect of raising Evasion. These effects can be broken with Brick Break, Psychic Fangs, or Defog, and can be overridden by Pokémon with Infiltrator. Does not stack with Aurora Veil, Reflect, or Light Screen, but can stack with Defense and Special Defense boosts. Non-Ice-types with Snow Cloak will take damage from Hail but gets the Aurora Veil effects.
Ice Body: Fire-, Fighting-, Steel-, and Rock-type attacks done to this Pokémon become not-very-effective attacks whenever Hail is active instead of its current effect of gradual restoration of HP. Does not affect Stealth Rock or Tinted Lens. Weakness-reducing Berries are not consumed under these effects. Non-Ice-types with Ice Body do not take damage from Hail but take super-effective damage as normal.
Stance Change (NERF): The user switches between Shield Forme and Sword Forme at the beginning of the turn instead of immediately prior to the move.
Gale Wings: The user gets +1 priority on Flying-type moves on the first turn in which it can attack after the battle begins or after switching in rather than being at 100% HP.
Mega Launcher: Instead of being 50% stronger, pulse moves always inflict a critical hit. Heal Pulse retains its effect of healing three-quarters of maximum HP instead of half.
Parental Bond (NERF): When this Pokémon uses a damaging move, the subsequent attack caused by this Ability has no secondary effects.
Schooling: If Wishiwashi is at Level 20 or higher, is in Solo Forme, and its HP increases to above 25%, it will revert to Schooling Forme right after its HP stops increasing instead of at the end of the turn. Has its regular effect if on a Pokémon other than Wishiwashi or a Pokémon transformed into Wishiwashi.
Gorilla Tactics (NERF): Damage is increased by 30% instead of 50%.
Cotton Down: The Speed drop caused by this Ability no longer affects teammates.
Screen Cleaner: Light Screen, Reflect, Aurora Veil, and Safeguard do not have any effect any time this Pokémon is in play instead of its usual effect of removing them when the Pokémon enters play.
Punk Rock (SEMI-NERF): Damage is increased by 40% instead of 50%. The accuracy of sound-based status moves has its accuracy increased by 20%.
Ball Fetch: This Pokémon has a 10% chance of picking up a Poké Ball all on its own after the end of a battle. The type of Poké Balls it acquires depends on its level, beginning with only the standard red Poké Balls at Level 1 and culminating with Apricorn Balls, Dream Balls, Beast Balls, and very rare Master Balls at Level 70 or higher.
Mimicry: In addition to type changes in terrain, this Pokémon also changes type from the weather: Water in rain and heavy rain, Fire in intense sunlight and very intense sunlight, Rock in sandstorm, Ice in hail, and Flying in fog (if that ever returns) and strong winds, including Tailwind. Once all terrain and weather effects end, this Pokémon resumes its default type(s).
Ripen: Leftovers heal twice as much HP as they normally would. Berry Juice is not affected by Ripen and no longer displays the message box indicating Ripen has taken effect.
