Technology Is Evil: "The Soul Sever" shows that Lion-O slowly adopted this point of view since the destruction of Thundera, going from one extreme to another. The events of the episode itself make him realize that technology isn't inherently bad or good, it's how it's used that decides it.
In "Omens Part 1": This leads Claudus to come to his unfavorite Lion-O's defense when Grune remarks on what a great king Tygra would have made within Lion-O's earshot, even though Claudus has expressed similar sentiments to Tygra himself.
In "Ramlak Rising" Lion-O discusses this in a fury over Tygra's agreement with Cheetara that avenging Claudus' death should wait. Instead, Lion-O wishes to hunt down Mumm-Ra, electing Revenge Before Reason.
This Is Gonna Suck: Cheetara's reaction when Panthro meets up with Grune for the first time since Grune's betrayal.
Threat Backfire: Grune destroys a tree in the Elephant's home hoping to scare them into telling him what he wants to know. They thank him kindly and honestly as removing the tree will give their gardens more sunlight now.
Threshold Guardians: In "The Trials of Lion-O" arc, the Spirit Stone makes Lion-O face guardians in the form of the other ThunderCats that challenge him to overcome his various flaws. Each guard keys that he must win through defeating their challenges, which then open gateways to proceed further in the trials, given to him to see if he is worthy of a second chance at life.
Through a Face Full of Fur: In "Journey to the Tower of Omens," Wilykit blushes briefly while making smooching noises, teasing Lion-O about Cheetara's Hands-On Approach when coaching him in the use of the Sword of Omens.
In "Into the Astral Plain", we see a young Tygra blush during a flashback to when he first met Cheetara.
In "Omens Part 1" During Grune's homecoming festival, while watching Gladiator Games in The Thunderdome, Lion-O finally has enough of Tygra's insults, and challenges him to a match, hoping to embarrass him in front of his hero, Grune. It doesn't end well.
In "The Duelist and the Drifter" this is how the Duelist gets young, Hot-Blooded swordsmen like Lion-O to agree to duel him. He favors the Showdown at High Noon.
In the same episode, the Drifter doesn't throw down a gauntlet so much as lazily proffer the willow reed in his mouth. He bets Lion-O can't split it, and gives him three swings to try...
Time Dissonance: The ThunderCats encounter the Petalars, who age in the span of a day - a briefly lost child reappears as an adult, and deeply touched that they had "never stopped looking for me" when he had been out of sight for mere minutes.
Took a Level in Jerkass: All the cats have done this at one point or another. Lion-O has an extended bout of it from "New Alliances" through the end of "The Trials of Lion-O," the first part of which had all the cats taking a level or two, actively picking on and subverting him. Justified in "The Trials of Lion-O, Part 1," as the other cats could see Lion-O was leading them on a wild goose chase into the trap that ultimately kills him (he gets better). They do feel pretty bad about it when he dies though.
As of "Native Son," Tygra's new whip, which was left to him by his biological father.
Trail Of Breadcrumbs: Wilykit, who seems to be the only one who believes that Lion-O is still alive in "The Trials of Lion-O, Part 1", marks some boulders with drawings of her own face as the group is captured and forced to go with Slithe.
Sometimes the Eye of Thundera is depicted with a black pupil in its resting state, other times, it's simply a red stone.
In "Old Friends" Grune's mace vanishes then reappears just as he's taking a swing at Panthro.
In "The Duelist and the Drifter," despite being narratively referred to as such, the Duelist's left hand sword does not match the gold Sword of Hattanzo, but is a gold recolor of the grey serrated sword the Duelist used in flashback before obtaining it.
In "Recipe for disaster", the swelling on Pumyra's face disappears and reappears throughout the first half of the episode.
The Unreveal: Mumm-Ra's attempted transformation in "Omens Part 2" is interrupted by the timely arrival of the sun.
Vagueness Is Coming: It's little wonder that Lion-O didn't know what his vision in the Sword of Omens meant. A contextless Mirror Monster image of a scary, red-eyed face doesn't exactly scream, "The Empire is doomed to fall on the 'morrow."
Vestigial Empire: Thundera, with its handful of living Cats, a monarch among them.
Vomit Discretion Shot: In "Ramlak Rising" when Tygra vomits off the side of the sand-ship, the viewer is spared the visual (but not the audio) through depicting him from behind, slumped over the ship's rail with his face off-camera.
The Soul Sever just wants to be reunited with his dead wife two children by putting their souls in machine bodies. It's a pity that he won't accept help from Lion-O after stealing the Book of Omens to complete the process, then tries to rip Tygra's soul from his body to merge it with a machine, and it all goes horribly wrong anyway.
Wham Episode: "Legacy". How did all the animal races get to Third Earth? They're descendants of the survivors of the crash of Mumm-Ra's spacecraft. Why is there Animal Jingoism? The Cats were Mumm-Ra's willing servants, the other species were slaves. Think the Eye is the only magical gem of power? Think again. And those are only some of the major plot points dropped in this episode.
"What Lies Above: Part 2". The leader of the Avistians changes sides mid-battle, shooting down Tygra. Mumm-Ra defeats Lion-O and knocks the Tech Stone free, thus causing the crash of the city. Pumyra is revealed to be Dead All Along and a Sixth Ranger Traitor who serves Mumm-Ra for Revenge and gives him the Tech Stone. Panthro manages to land the city mostly safely, and the kits succeed in showing Lion-O the good he has done through all those who came to ally with him, but the battle against Mumm-Ra has become even more dire with only one stone left to find and Lion-O's heart and faith are broken.
Pumyra: Lion-O, you are my king... but he is my master.
What Measure Is a Non-Cute?: In "Survival Of The Fittest", the hungry kittens refuse to kill the cute prey, but they gladly beat up an ugly beastly predator (who was hunting to eat) to rescue them.
What the Hell, Townspeople?: In "Omens Part 1" the Thunderian Angry Mob is foolish and antagonistic enough to pick a fight with Lion-O and Tygra, their princes, one of whom will eventually rule over them.
Frequently directed at Lion-O. In "Omens Part 2" Tygra chastised him for not saying anything about his vision of Mumm-Ra.
In "Ramlak Rising" Tygra and Cheetara call him out for nearly driving them to ruin when consumed by rage.
In "Old Friends" Panthro cannot fathom his impulsive blind charge into battle.
Whole Episode Flashback: "Old Friends," the bulk of which reveals Panthro and Grune's history, Grune's motives and betrayal, and why Panthro was believed dead. note First hinted by the art director here in an early review.
In the pilot, the Thunderkittens pass near a transport filled with gold and jewels looted by the Lizards from the Thunderra treasuries, and prudently decide that taking the clothes lying among the gold will be the most practical choice.
In "The Duelist and the Drifter", Lion-O tries to pay with Thunderran currency, but finds out to his chagrin that its value has dropped significantly since the kingdom's fall.
Xanatos Gambit: The Siege has multiple traps to force Thundera's fall. First Claudus and Tygra are trapped at the gate and offered Panthro in exchange for the Sword of Omens. When Jaga's Clerics intervene, they're led into a Defensive Feint Trap in The Thunderdome, surrounded by Walking Tanks, until Lion-O intervenes. When they get to Panthro, he stabs Claudus, reveals himself as a disguised Mumm-Ra, and kills the Clerics. "Thundera has fallen!".
Lion-O: It's official, they think I'm a failure. And they always will unless I can prove I'm not chasing a childish dream... But how can I do that when even Grune said there wasn't tech out there?! Jaga: Don't let what Grune failed to see stand in the way of what you believe.