Mazinger Z provides examples of the following tropes:
Jack Bauer Interrogation Technique: Discussed in episode 56. The main characters capture two Mooks, and Nuke and Mucha decide interrogating one of them using the fearsome procedure of... tickling it to death. Boss ran out of patience, though, and stated that it was time to use real torture techniques. However that got Sayaka utterly scandalized and she berated him for even thinking about it, stating that war prisoners have human rights, too. Boss relented.
Japanese Ranguage: Due to this trope Baron Ashura was called Baron Ashler in the Spanish dub. And sometimes Count Brocken's name is mispronunciated like Blocken. Oh, and Dr. Hell's name is written "Heru".
Jet Pack: Mazinger-Z's Mid-Season Upgrade was a Jet Pack that docked with Mazinger-Z. It was yellow with red Razor Wings and could shoot star-shaped shurikens to whoever tried attack Mazinger-Z from behind. Its name was Jet Scrander.
Joshikousei: Sayaka in the Go Nagai manga (in the anime she did not wear school uniform). However she wears a modest-length skirt (that, given who created her, it is ironical).
Jumped at the Call: Kouji Kabuto. After his grandfather's murder, Kouji was just eager to pilot Mazinger-Z and avenge him. In Shin Mazinger Zero we saw what could have happened if he would have Refused the Call. It was... not-pretty.
Just a Machine: Subverted. Kouji and his friends usually felt no remorse when they blew up giant robots. But when they destroyed a Beast that acted more like an human being than a machine, or when a Ridiculously human Robot died, they often felt sad. When Kouji killed the Gamia sisters, they were so human-looking he felt sickened and disturbed. Dr. Hell and his Co-Dragons nearly always regarded his robotic soldiers like Just Machines and disposable, but there are exceptions: Baron Ashura called Gamia Q1, Q2 and Q3 his/her "daughters", and he actually grieved their deaths (and mind you this is the thing who is capable of machine-gunning between laughs a group of survivors of a shipwreck it caused).
And then you have Minerva-X. She was capable to think, feel and act on her own. Kouji and his friends treated her like if she was a person and kouji went so far to bury her after her death.
Kangaroo Court: In one episode Baron Ashura had trapped Kouji and decided "judging" him, playing judge, jury and executioner.
Kick the Dog: Sort of averted. Baron Ashura is not above doing pettty acts of cruelty, but usually he does them with a purpose in mind. Still, some of those acts include: hitting children on the head to knock them out, blowing up the wheelchair of a paralytic little girl to prevent her from running away, ordering one of his Iron Masks to kill a little cub whose mother had just got killed...
Kid Hero: Kouji Kabuto was sixteen when began the series. Sayaka, Boss, Nuke and Mucha were the same age, and Shiro was roughly ten-years-old.
Kid Hero All Grown-Up: Kouji is sixteen at the beginning of the story. When he comes back to fight the Mykene army, he's eighteen. When he allies himself with Duke Fleed to fight the Vegans, he's nineteen.
Kinetic Weapons Are Just Better: Mazinger-Z was the first Humongous Mecha -how many times have we repeated that sentence?- that used Eye Beams. Photon Beam was the weapon Kouji used most often, and many Mechanical Beasts were equipped with laser beams. However, Mazinger-Z also used missiles, as did most of Mechanical Beasts. And Dr. Hell was more than capable of building beam weapons for his troops, but his minions nearly never used them. His Co-Dragons and Mooks prefered guns or machine guns that shot regular bullets.
Lap Pillow: In chapter 38, Mazinger-Z rests his head on Minerva-X's lap while Kouji talks with her about her nature -and the fact that she's a sentient robot and is in love with Mazinger-Z-.
Latex Space Suit: Unsurprisingly, one of the earliest examples in Anime. The pilot suits for both male and female characters. Made with Alloy Z, they are designed to protect them from harm. Kouji began to wear a Latex Space Suit in the third episode, and Sayaka always wore one while piloting Aphrodite A. Sayaka's was one-piece and tighter than Kouji's though.
Justified Trope <=> Unbuilt Trope: Kouji did not wear one in the first chapters, and often he got badly hurt, bruised or wounded. So a skinsuit was made for him in order to protect his body.
Lava Adds Awesome: Mazinger-Z's Home Base was located on the foot of Mount Fuji. Such a perfect setup was used for animating many battles and squirmishes would have been less cool without the Climactic Volcano Backdrop, the characters brawling around a Lava Pit or soaring above Mount Fuji's crater. In one episode, Dr. Hell attempted to set an eruption off to bury the Photon Research Institute beneath tons of lava. In another episode, Mazinger-Z was tossed into a volcano. In the Gosaku Ota manga episodes, The Dragon Baron Ashura's master plan involved to set volcanic eruptions off throughout Japan...
Lava is Boiling Kool-Aid: Several times Kouji fought near flowing lava. Since he always was inside his Humongous Mecha it is Hand Waved that Mazinger-Z's armor and insulation were protecting him from the extreme heat, at least for a while. Still, in one episode he got dunked INTO a volcano. The characters pointed out, that not even Mazinger could endure THAT for very long, and they had to get it out of the magma RIGHT AWAY or Kouji would die. Nevertheless, usually the lava looks clearer and more liquid than it should be (although it is somewhat more viscous than in other examples).
Lava Pit: Several Mechanical Beasts were fought beside or over lava pits on the crater of a volcano -usually Mount Fuji-: Aeros B2 and B3, Holzon V3 -in one of the manga versions-, Debira X1... And in one episode, Kouji was dumped in one.
Lensman Arms Race: Throughout the series, Mazinger Z has to be constantly upgraded and endowed with new weapons and capabilities in order to battle Dr. Hell, increasingly powerful robots. Of course it drove Hell to create still more powerful Robeast and when Mazinger Z finally could not catch up, it was replaced with an entirely new robot, Great Mazinger.
Like an Old Married Couple: As Kouji and Sayaka are arguing -and fighting- in the background, Shiro is fortune-telling. He notes the cards tell his brother and Sayaka are destined to be together forever.
Long Hair Is Feminine: Most of the female characters wore long hair: Sayaka, Misato, Erika, Hitomi, the Gamia sisters... Sayaka even wore Hair Decorations (a pink headband). Of course, it was considered a girl seemed more femenine like that.
Long Runner: Forty years after the inception of the series, more Mazinger Z-related manga, anime, games and toys are being made.
Longing Look: Sayaka to Kouji, constantly. Kouji's reaction depended on the version: in the anime, most of the time he did not notice or he teased her about it; in the manga, he was nicer about it (the anime adaptation made him bigger of a jerk).
Disturbingly, Big Bad Dr. Hell did this constantly to Yumiko Shiozai -Kouji's grandmother- the whole time (according his Back Story narrated in the Gosaku Ota manga episodes). Of course, he represented the stalkerish verison of the trope. Yumiko either didn't notice or pretended she didn't.
Lost Superweapon: Mykene's Mechanical Beasts. Old myths assured an ancient, allegedly lost Greek civilization called Mykene lived on the island of Bardos used metallic giantsthat shot flames from their chests to defend their land. Dr. Hell hypothesized that maybe the myths might be true. Unfortunately for everybody, the old legends were indeed true, and he found an army of ancient, forgotten Humongous Mecha under the ruins of the island. However it is a subversion, since as it was seen in Great Mazinger, the Mykene civilization still existed, and throughout millennia had dramatically improved their technology, and compared to their newest mechas, the giant robots Dr. Hell found were ancient, outdated, mountain sized piles of scrap.
Lost Technology: The plot is set in motion when Dr. Hell finds an army of giant robots in the underground mazes of the Greek island his archaeological expedition was researching, belonging to the lost Mykene civilization. Instantly he decides to seize that technology to further his goals. Unfortunately for him -and the world- the legitimate owners of that technology were still around...
In the sequel Boss also chased after Jun, who was aboslutely fixated on Tetsuya (and Tetsuya was obsessed with fighting, and he seldom showed he reciprocated).
Kouji also found attractive some girls in the series (Hitomi, Erika, Misato...) and Sayaka was very jealous when it happened... including the situation with Minerva-X, a sentient mecha who loved Mazinger-Z.
In the other sequel, Hikaru is in love with Duke. Duke may reciprocate her feelings. Rubina is in love with him. Duke certainly reciprocates her feelings. And Minister Zuril was also in love with Rubina. And Kouji kept a Belligerent Sexual Tension relationship with Maria. Who knows what would have happened if Sayaka and Maria would have met in the original series.
In the Dynamic Heroes e-manga they met, and neither of them appreciated when one of them got very close to Kouji.
Love Makes You Crazy: Dr. Hell can be a subversion because he was already quite unstable and troubled before... but when he was in college and fell in love with a cute Japanese exchange student he started behaving obsessively (getting angry if someone dared to spend time with her) and erratically (his grades started slipping because he was too obsessed to study). And when he found out she was in love with another person, he flipped out completely and tried to Murder the Hypotenuse (and even stormed into the campus with a shotgun!). By the way, that woman was Kouji's grandmother.
Boss also tended to act in a very dumb, irrational and even obsessive way when he was in presence of Sayaka or Jun Hono. Sayaka tended to ignore it, but Jun did not appreciate it or find it amusing.
Love Makes You Evil: Dr. Hell fits the trope to the letter. He was psychologically unstable before... but when he fell in love with a cute Japanese student attended his college he became obsessive, jealous (he even fumed if she spent time with someone else) and erratic. When he found she was in love with someone else, his mind finally snapped out completely. He thought everyone was out to get him, and attempted to Murder the Hypotenuse (he stormed into the campus with a shotgun!). That incident (and another where he tried to help someone and he got the crap beaten out of him for it) was his Start of Darkness. He decided Humans Are Bastards and he would make everyone pay. People had shunned him out of scorn or indifference before, but from that day they gave him a wide berth out of fear because he already started looking Obviously Evil (and downright creepy).
Luke, I Am Your Father: Subverted. In episode 90, a woman showed up in the Institute, claiming being Kouji and Shiro's mother. It is subverted because in reality she was a cyborg, pretending being their real -and very deceased- mother. This trope was nearly played straight when Kenzo Kabuto showed up, but Kouji and Shiro would not find out about it until the next series.
Luminescent Blush: It did not happen in the anime often (since Kouji was supposed to be a Chaste Hero), but it was a fairly normal occurrence in the manga episodes by BOTH Kouji and Sayaka.