Abnormal Ammo: In "Berbils," mercenary slaver the Conquedor wields a BFG that fires giant, brightly-colored globs of adhesive goo to stop attacks, while his Cute Machine victims the Ro-Bear Berbils later retaliate by building a green slime dispensing turret gun in defense of their village.
Absurdly High-Stakes Game: In "The Duelist and the Drifter," when Lion-O challengesMaster Swordsman the Duelist to a fight, and raises the stakes to winning the Duelist's collection of trophy blades, the Duelist insists that since Lion-O has only one sword to offer in return, the boy should agree to give up his life if he loses. Lion-O agrees to the terms.
The Drifter: "This is a Swordsmans' town, stranger, and they duel for keeps. Leave, before it's too late. Or don't, I don't care."
Adventure Towns: While not always towns in the strictest sense, the ThunderCats travel somewhere new every episode.
In "The Duelist and the Drifter" The Swordsmans' Town is the first real city Lion-O sees outside of Thundera. Even before entering, he is warned of the danger found therein, particularly for young, cocksure swordsmen like himself. Lion-O likes adventure.
In "Berbils," the Ro-Bear Berbil village seems a sweet Sugar Bowl, until the ThunderCats bear witness to slavers kidnapping the villagers.
In "Sight Beyond Sight" an elephant village is plagued with a wasp problem the ThunderCats try to solve.
"The Duelist and the Drifter": Power alone is rigid; you must be flexible to succeed.
"Between Brothers": Do not confuse a preference to avoid conflict with weakness or pacifism.
Aesop Amnesia/Broken Aesop: In "Song of the Petalars," Emrick (questing to restore his people to their homeland) impulsively confronts a bird so large he's outmatched and Lion-O must save him. Lion-O complains of his teenaged stupidity. Later, Lion-O (questing to save his people and their homeland), in his teenaged stupidity, impulsively confronts enemy forces so large he's outmatched and a Deus ex Machina must save them. When Lion-O attempts this again in "Old Friends" Panthro quickly loses his patience.
In "The Pit", Pumyra is on the fence about trusting Lion-O until he nearly gives his life to save her. In "The Curse of Ratilla", Pumyra is on the fence about trusting Lion-O until he nearly gives his life to save her. In "Birth of The Blades"...well, just guess. Of course, this one may be justified, knowing what we know now.
Alien Blood: In "Ramlak Rising" the Giant Squid/Man-Eating PlantHybrid Monster the Ramlak is shown to have a strange blue ichor dripping from the severed ends of tentacles that Lion-O slices off when it first surfaces from the Sand Sea and attacks. When Lion-O strikes a mortal blow, its revealed that the monster's blood is, appropriately enough, water.
Alien Sky: Third Earth orbits a gas giant with a great spot visible on its surface. Two moons are also visible. (Third Earth is implicitly a moon of said gas giant)
Alike and Antithetical Adversaries: The series begins with homogenous Catfolk heroes versus a heterogenous group of villains, but the ThunderCats meet other races over time, coming to realize that such alliances are necessary to defeat Mumm-Ra.
All Crimes Are Equal: Downplayed. When Lion-O attempts to shame a Powderkeg Crowd of Thunderian townspeople by announcing that stockaded Lizard scavengers "don't deserve this [harassment]," their pointman replies: "These barbarians deserve death!" And with that, an Angry Mob is born. The surprise comes when King Claudus is incredulous at Lion-O's attempt to stop them.
All of Them: In "Between Brothers", when discussing the Lizard army that has surrounded the Elephants' village.
Panthro: Bad news. The entire village is surrounded.
Cheetara: How many troops?
Panthro: If I had to guess, I'd say "All of them."
All Planets Are Earth-Like: Deconstructed with Third Earth, which, while superficially similar and suspiciously habitable, is just as often a fantasy world with many organic structures deliberately mixed-and-matched or given a Mouse World level of detail, as art director Dan Norton explains here.
Always Someone Better: Tygra wins and loses as much as any other member of the team, but the series makes a point of the fact that Lion-O never defeats him, regardless if it's in the real world, the Astral Plane, or his own mind.
When "The Duelist and the Drifter" reveals that more unconventional Humanoid Aliens also populate Third Earth, pink, yellow and purple-skinned creatures appear, most notably the lavender-complected Rubber Forehead Alien the Duelist.
And This Is for...: When Tygra thinks Lion-O’s dead (which he is) he punches the ones who did it while saying “That was for me. And that was for Lion-O”
Androcles Lion: In "Omens Part 1", Lion-O saves a pair of stockaded Lizard prisoners from being lynched and frees them. Later, during The Siege of Thundera, the more antagonistic of the two repays the favor by slipping Lion-O the key to his and Tygra's cell.
Animal Eyes: Thunderian Cats have slitted pupils, but visible sclera, while Lizards have fully yellow eyes with slits.
Animesque: Though the original series' Rankin/Bass also outsourced production to Japan, the reboot's look and animation by Studio 4°C are now explicitly touted as major selling points. The Snarf short "Butterfly Blues" even proudly bears the stamp "Made in Japan!"
Apocalypse How: Mumm-Ra causes an X-3 in the backstory in order to create the Sword of Plundarr.
Arbitrary Skepticism: Though itself a magical kingdom, most of Thundera outright dismisses technology as the stuff of fairy tales, and are skeptical of the existence of The Book of Omens and Mumm-Ra. Justified in that Thundera is depicted as fairly isolated, and their history has fallen into myth.
In "The Curse of Ratilla," Tygra insists that the titular curse is just superstition meant to keep the slaves in line, despite a long string of near-fatal "coincidental accidents" during his and Cheetara's search for the Sword of Plundarr. Never mind that he's seen firsthand that curses are real in "Native Son."
"Into the Astral Plane" explores the childhoods of Cheetara and Tygra, including how the former became a Cleric
"Native Son" reveals Tygra's origins, and how he came to be adopted by Thundera's ruling family.
"Survival of the Fittest" reveals Wilykit and Wilykat's past, and the reason why they are looking for El Dara.
Badass: They're everywhere! Panthro, Grune and Claudus especially.
Badass Adorable: Lucy, from the episode Recipe for Disaster. A large caterpillar, and later butterfly, used as a beast of burden by a supporting character. Cute as a bug's ear. Goes toe-to-toe with Mumm-Ra, who has taken the form of a giant, armored dinasaur, and kicks his newly-scaly ass.
Balcony Speech: King Claudus gives one in a public arena dedicating some celebratory games to the memory of Panthro.
Barefoot Cartoon Animals: The entire Animal cast so far, except Leo, Panthera and the Swordmaker. Justified for Cats, who can use their foot claws to climb structures like the giant tree featured in their games. Most Cats make a point of wearing spats for fashion's sake, but there are exceptions such as Pumyra.
Bat Signal: In "Song of the Petalars," this appears as an empathic power of the Sword of Omens. Lion-O's By the Power of Grayskull! causes the Eye of Thundera to project his royal emblem into the sky just before Lion-O and his ThunderCats enter a battle Which brings a nearby Panthro to the rescue.
Battle Couple: Cheetara and Tygra. Lampshaded in "What Lies Above, Part 2." Cheetara to Tygra as they battle with Addicus and Slithe: "Is our whole relationship based on saving each others' lives?"
Jaga: "For it was written that he would be born of fire, a king to lead his people to victory, against ancient spirits of evil."
Bedsheet Ghost: In "The Forest of Magi Oar," the Thunderkittens tease Cheetara when she reports feeling the presence of spirits by playing at being ghosts under a tent canvas. While they're stumbling under it, they knock their heads together.
Deconstructed in the same episode when Lion-O and friends try to save some Lizard prisoners from abuse, only to trigger lynch mob sentiments and a brawl which Claudus has to stop.
Played straight in "Omens Part 2" with Claudus and Tygra saved by the Clerics, then by Lion-O, when surrounded by Walking Tanks.
In "Song of the Petalars." the ThunderCats are saved from a presumed Last Stand against the Lizard army by Panthro in the Thundertank
Played straight again in "Into the Astral Plane" when Tygra roars to the scene to save the Cats in the Thundertank, without Panthros's permission. Tygra doesn't let Lion-O forget it, either, and lampshades it himself later in the episode.
Subverted in "New Alliances" when Lion-O, Cheetara and Tygra try to liberate lizard soldiers from Mumm-Ra's army; instead, they get owned by the villains Addicus, Kaynar, and even Slithe.. Played straight again later in the episode when Panthro has to come bail out the trio in the Thundertank again.
Big moment with Lion-O in "The Trials of Lion-O part 2" where he comes back from the dead to save the Cats and sends Mumm-Ra ducking for cover.
Black Box: Lion-O purchases a piece of what he suspects is Lost Technology from a Friend in the Black Market, and spends quite a bit of time puzzling over it and diagramming it, but he only realizes its function when he sees a Lizard use one to blow up a wall.
Black Market: Lion-O apparently makes a habit of traveling through Thundera's back alleys to frequent his friend Jorma's shop in search of "certain hard-to-find collectibles". Cheetara assumes the wares are forgeries.
During The Siege of Thundera. A sky full of arrows, Claudus cutting through a wave of Lizards, even Claudus getting stabbed in the back and falling into pool of water, not one drop of blood is found.
Played with in "Song of the Petalars" the Cats cut and shoot through Lizard troops bloodlessly, but Tygra's shots are shown to pierce the Lizard's bodies.
In "Between Brothers", Panthro's arms get caught in the collapsing Astral Plane; when the smoke clears, the severed stumps are covered in glowing blue energy.
In "Native Son", a character is stabbed by Tygra's father, Javan, but after the knife is pulled out there isn't a drop of blood on it. Though in this case, it's possibly somewhat justified as it turns out that Tygra's clan were Dead All Along.
Building Swing: In "Omens Part One" The jungle vine variant, employed to knock another player off a giant tree and into a pool of water below, is briefly seen as the Thundera's Gladiator Games are introduced.
Bullet Seed: Or rather, bullet pollen. The Petalars' method of attack.