Novelty Decay

In many series, something or someone is first introduced as special - new, awe-inspiring, mysterious, utterly unique, unparalleled, or some combination of those things. Sometimes, either later in the series or in related works in shared universes, that specialness seems to fade without much explanation or get outright retconned away. The unique example becomes just one of many, the mystery somehow gets thoroughly documented, the new arrival turns out to have a long history in the area, the unparalleled turns out to be a footnote, and the awe becomes...ehn.

This is a form of Continuity Drift, perhaps sometimes due to They Just Didn't Care or Did Not Do the Research on Long Runners. It can be justified if enough time passes and the once unique aspect is spread due to analysis/teaching/reverse engineering.

Anime and Manga

 * The titular magical orbs of Dragonball Z start out as the awesome Holy Grail of all mystical artifacts, then eventually become relegated to fuel for resurrections as the series continues.
 * From the same series is the Super Saiyan. Goku used to be the only one in existence. Now there are around 9 Super Saiyans, along with at least 3 different levels of transformation. Any hero without this ability is pretty much Overshadowed by Awesome.

Literature

 * Star Wars:
 * The rancor beast from Return of the Jedi was originally described in the novelization as a mysterious, possibly mutated creature: "The size of an elephant, it was somehow reptilian, somehow as unformed as a nightmare. Its huge screeching mouth was asymmetrical in its head, its fangs and claws set all out of proportion. It was clearly a mutant, and wild as all unreason." Nevertheless, due to rancors being awesome, they quickly proliferated through the Expanded Universe. Later the novel 1994 novel The Courtship of Princess Leia introduced the previously undiscovered planet of Dathomir, to which the rancors are native. Dathomir itself then propagated in the EU, to the point that the 2004 novel Ruins of Dantooine included Dathomirian beasts, a common biologist with detailed knowledge of Dathomir's ecosystem, and other characters who'd apparently been to the planet ... 8 in-universe years before the planet became known beyond a small circle of individuals.
 * Grand Admiral Thrawn was initially presented as a member of an unknown species, established after 8 years OOU and 10 IU timeline years as the heretofore reclusive Chiss species from the Unknown Regions. After this, of course, Chiss proliferated in the EU, most recently and egregiously in the novel Darth Bane: Path of Destruction a full 1000 years before the species was supposed to have been known to the galaxy at large.
 * The most recent and even more Egregious example now is The Old Republic, where The Chiss are now running around over 3,500 years before they're officially supposed to be known. Both this and the previous example can be brushed off by them not being hugely known and keeping largely out of the popular mind, and the galaxy being kinda large, but the trope is still there.
 * Obi-wan Kenobi and Yoda were intended to be the last surviving Jedi from the old Jedi Order, period. Of course, it didn't take long for the Expanded Universe authors to start creating their own survivors who went into hiding or were mysteriously overlooked. Not to mention the hundreds of Sith all over the place.

Live Action TV

 * The Borg's first appearance was on Star Trek: The Next Generation as a mysterious, frighteningly advanced and implacable species from beyond known space. Then Voyagers premise is to travel through said quadrant to return home, frequently defeating them and even allowing an individualised one onto the USS Voyagers ranks.
 * In the early years of Doctor Who, we knew almost nothing about his people - it was six years before we learned the name "Time Lords". From the Tom Baker serial The Deadly Assassin onwards, we began to learn more and actually visited Gallifrey. Over the next decade or so, more stories featuring the corrupt, self-interested and machiavellian Time Lords were made, to the point where many fans complained that too much was being explained and the mystery had gone. One of the objectives of the so-called "Cartmell Master-plan" was to Retcon some of this and reintroduce the mystery.
 * The new series managed to bring this back a bit, by having them all supposedly killed off. This means that for modern fans, any slight suggestion that there might be another one out there is incredibly exciting.

Comic Books

 * Superman was supposed to be the last of his kind, and kryptonite, the radioactive fragments of a planet that blew up lightyears away that didn't burn up in our atmosphere, was a rare commodity almost nobody had heard of. Throughout the Silver Age, a veritable rainbow of different sorts of kryptonite showed up, as did first Supergirl, then other survivors of Krypton. After the post-Crisis on Infinite Earths reboot, Superman was once again the last Kryptonian and kryptonite one troublesome and very rare material. ...And then, after a few years, the other Kryptonians started showing up again, along with the wacky varieties of kryptonite.
 * Daredevil: Matt Murdock's radar sense was original a super power, the freak result of the accident that blinded him. Later, Frank Miller introduced a mentor character and revealed that anyone could learn to "see" without their eyes the way that Matt does.

Music

 * Technical abilities displayed by musicians have gotten increasingly advanced in recent years, making formerly advanced musicians less impressive by comparison. The technical skills of Jimi Hendrix are within the expected reach of intermediate-advanced guitarists compared to modern guitarists like Tosin Abasi or Muhammed Suicmez. Similarly, previously high-speed drummers from a few decades ago are dwarfed by modern players, with extreme metal drummers like George Kolias or Jon Rice frequently utilizing eighths and sixteenths in excess of 240bpm.

Toys
Gold went through an easier course. While beginning from 2004, many sets had it as their secondary or tertiary color, there were always figures (often special edition ones) that gave it some uniqueness. And in 2010, a former gold-wearing character had to be recolored silver just to make that year's "Golden Bionicle" promotion all the more special. But this didn't make all the other, non-special golden characters nonexistent.
 * This appears frequently Bionicle, often overlapping heavily with the Doing in the Wizard tendencies it develops near the storyline's end. Almost every detail has to have a subversive explanation behind it. As the writer described to the fans, tease all the plot threads you want, but you have to deliver the explanations sometime.
 * The original six Toa were introduced as borderline-mythical, ultra-powerful warriors. The Toa. When the Seventh Toa appeared, it was some truly major thing that heavily affected the plot. Then came the revelation at the beginning of the Metru Nui saga: "You are not the first Toa!" During that arc, one scene had a total of 300 Toa appearing at once. Since then, Toa have became as "standard" as the Jedi in their respective universe, and the original six had to be re-branded as Toa Mata.
 * Likewise, the Makuta, nowadays known as Teridax. The most mysterious and powerful, nearly untouchable evil might, the Master of Shadows and the ultimate foe the Toa have to face. Subversion: no, he's just one of the original 100 Makuta. Double Subversion: Makuta in general and Teridax in specific is still a tough bastard to beat, and what he lost in novelty, he makes up for with his schemes. But Makuta, as a species, are nothing special now.
 * Silver colored pieces. To a lesser degree, also gold. When the Toa upgraded into Toa Nuva, they received silver armor and weapons to reflect that they're "more" than mere Toa. The Bohrok-Kal came along, sporting silver as their secondary color to showcase their eliteness compared to regular Bohrok. After these, silver parts became a standard for absolutely every character, and colored weapons became such a rarity in fact, that when the 2009 first wave sets re-introduced element-specific colorings, there was much happiness... that lasted 'till the second wave, which went back to giving silver weapons to about half its sets.

Video Games

 * The Chaos emeralds from Sonic the Hedgehog are an inversion. Their original purpose was just to unlock the Good ending, but from the second game onwards became Plot Coupons that unlocked the titular character's Super Mode.

Western Animation

 * In The Legend of Korra, the rare and unique Elemental Powers or abilities of Avatar: The Last Airbender have become commonplace. Lightningbending, which was emphasized as being dangerous and masterable by only a select few, is so common people who need low-paying second jobs can do it to run a power plant. Similarly, metalbending (previously exclusive to the original series' Toph) has an entire police squad of practitioners (though this is not an example of Continuity Drift, as all known metalbenders were either trained by Toph or by someone trained by Toph). Finally, the Equalists use Ty Lee's Pressure Point-based chi-blocking techniques to fight against benders.