Later series introduced cranial ridges, and Discovery created a more alien look for the Klingons. Genetic modification is explored as a possible explanation for the appearance differences, with Enterprise featuring a storyline involving a virus that caused Klingons to lose their distinctive features.
While the reason for the discrepancy between The Original Series Klingons and their feature film and later television series counterparts was a lack of budget, fans took it upon themselves to contrive an acceptable canon reason for the sudden change.
DIS Klingons were an isolated sub-species that vanished after DIS. In season 1, the focus is on T'Kuvma and his closest allies. They all share the rare traits, while the actual majority of Klingons, the ones that might look as in TMP or even in TOS, are what seems to be a minority in Discovery.
These Klingons had full-on monster heads, clawed hands, and were almost universally, bald. Discovery later suggested that the Klingons were only bald because they had shaved their heads for war and in Season 2 L'Rell (Mary Chieffo), and other Klingons grew their hair out.
Most notably the make-up of the Klingons was modified for TMP. In TOS their foreheads were flat exactly like those of humans. The only distinguishing mark was their beards. In TMP and thereafter, however, the Klingons had ridges on their foreheads, to emphasize their being alien but probably also their martial nature.
Star Trek: The Next Generation Season Two Blooper Reel
Why does Worf look different?
After season 1 of TNG wrapped production, Worf's facial prosthetics were either lost or stolen. This presented Michael Westmore, TNG's primary makeup designer, with an opportunity to improve the look of the character.
Producers brought in a Klingon language expert to ensure the dialogue was in keeping with the fictional language created for the Star Trek: The Original Series cast's film created by Marc Okrand. The heavy makeup and the vocal effect, unfortunately, made the dialogue sound muffled by the prosthetics.
In Star Trek VI: The Undiscovered Country (set around 70 years before the Next Generation era began), Colonel Worf (also portrayed by Dorn) appears as the legal advocate of Captain James T. Kirk and Dr. Leonard McCoy after they are accused of killing Chancellor Gorkon of the Klingon High Council.
The redesign of the Klingons almost from scratch for Star Trek Discovery is the arguably most controversial artistic decision in the history of the franchise. Whether they criticize the new look and feel of the species for its clumsiness or for its disrespect of canon, many fans genuinely dislike it.
Why do Klingons look different in Discovery season 2?
Star Trek: Enterprise conjured the Augment Virus to justify the changes to the Klingons, but Star Trek: Discovery's revamp of the Klingons never got any kind of explanation. It was merely hinted, as the Klingons grew their hair out in Star Trek: Discovery season 2, that they would eventually look like TNG's Klingons.
What happened to the Klingons between the original series and the next generation?
Very simply they look different for the same reason that the Klingons on The Next Generation, Deep Space Nine and all the later series made them look different: Modern makeup is much cheaper and easier to apply than that used in the 1960's.
In Star Trek III: The Search for Spock, Kirk's son David Marcus was killed by Klingons, and it hardened Kirk's heart. He hated Klingons for it and was furious when Spock set him up (three movies later with Star Trek VI: The Undiscovered Country) as the escort and emissary for Klingon Chancellor Gorkon.
Being descended from DNA molecules planted by the Ancient Humanoids, Klingons are distantly related to the other humanoid species of the galaxy such as Humans, Romulans and Cardassians, even though each evolved independently on their respective homeworlds.
Memory Alpha notes that there is no canonical maximum estimate for Klingon lifespans, but several Klingons are known to have exceed a century, and some may have made it past 150 (earyh) years. That makes them pretty comparable to 24th century humans.
Star Trek: Lower Decks season 4, episode 9 reveals that Klingons have pink blood, a departure from their usual red blood. The choice to give Klingons pink blood was made in Star Trek VI: The Undiscovered Country to create an alien look and to avoid a higher film rating.
Klingons are known to have sexual relations with humans, with the character B'Elanna Torres being an example of a Klingon-human hybrid; Torres' mother is a Klingon.
Picard replied with the phrase, "Qu'vatlh ghuy'cha' baQa'," at which point Vagh complimented him on his mastery of Klingon cursing. Qu' (task) and vatlh (hundred) make up the basis of the word Qu'vatlh (pronounced); thus, "a hundred tasks," frustration with an excessive workload and a lack of appropriate resources.
A small number of people are capable of conversing in Klingon. Because its vocabulary is heavily centered on Star Trek-Klingon concepts such as spacecraft or warfare, it can be hard for everyday use because of the lack of words for a casual conversation.
Worf was Married to Jadzia Dax and (spoiler) at her death in DS9 became a widower. He did have a fling with Deanna Troi through 2370, which was the result of Worf seeing himself Married to Counselor Troi in an alternate reality which led him to actually pursue Deanna romantically which was welcomed by her.
Picard was under severe emotional distress, but the crew was unwilling to call him out on it - except for Worf. When he expressed his concerns to Picard, he was called a coward - a huge insult in Klingon culture. Worf became visibly angry, and told Picard he would have killed him if he had been anyone else.
Later in the series, she becomes involved with the Klingon character Worf, and they marry during the sixth season of the show. In the sixth-season finale Jadzia dies but the Dax symbiote survives, and in the seventh-season premiere returns with a new host Ezri Dax, played by Nicole de Boer.
Why do Klingons look weird in Star Trek Discovery?
The appearance of Klingons in Star Trek has evolved over time, with the original series using minimal makeup due to their tight budget. Later series introduced cranial ridges, and Discovery created a more alien look for the Klingons.
The Klingon death ritual was a Klingon rite performed during, or directly following, the death of a warrior. The Ritual involved opening and staring into the eyes of the dying individual, then bellowing loudly at the sky. The former served to observe and confirm the lack of recognition by the dying/dead Klingon's eyes.
Worf is an outsider to Klingon society, marked as different both by his positionally — adopted child of humans, member of Star Fleet — and by his unshakeable commitment to the values that Klingons espouse.
