Gohan could've been Dragon Ball Z's greatest character, but he ended up being one of its biggest wasted opportunities instead. Introduced at the very outset of Goku's adventures as an adult in Dragon Ball Z, Gohan is the protagonist's firstborn son and as a shy, cautious child, is the polar opposite to his impulsive and forthright father. However, Gohan possesses a hidden, latent power as the result of his mixed Saiyan and human biology and these abilities bubble away beneath the surface, threatening to spill over uncontrollably. Increasing in power and prominence as Dragon Ball Z progresses, Gohan finally fulfills his potential by becoming the first Super Saiyan 2 and defeating the arch-villain Cell in an epic clash.
Dragon Ball creator, Akira Toriyama, has confirmed that his initial plan was to have Gohan take over as main character at this point in the story, but whether due to the author's own misgivings or pressure from publishers and studios, that plan was nixed. Instead, Gohan became the hapless, camp superhero known as the Great Saiyaman - an idea roundly panned by fans. Goku's son enjoyed a significant power-up in the Majin Buu saga, and ended Dragon Ball Z as the strongest unfused character, but he also failed to play any kind of major role in the series' big final battle.
Gohan's story might've suffered somewhat after he was dropped as the lead, but he at least enjoyed some standout moments in Dragon Ball Z's final arc. The character's real fall from grace has occurred over more recent years, during Dragon Ball's renaissance on the big screen and in Dragon Ball Super.
Following the defeat of Buu, Gohan should've still been Dragon Ball's strongest fighter in the Battle of Gods movie but, instead, he became a full-time scholar whose fighting skills had rusted almost entirely. After being scaled down in power, Gohan is left to fade into the background along with other abandoned Z-Fighters such as Yamcha and Krillin. Considering so much focus was placed on Gohan in Dragon Ball Z, this is not just a backwards step for the character, but almost a complete reset, going back to the days when Gohan would fight reluctantly, but ultimately had to leave the real business to his Dad.
This downwards slope has continued in subsequent movies and TV series, with Gohan routinely humiliated and made light of. Gohan hardly features in modern Dragon Ball stories and, when he does sporadically appear, it's usually in the capacity of a family man or someone tentatively reentering the fighting world. In the Dragon Ball Z: Resurrection F movie, Gohan even struggles to attain his standard Super Saiyan form - something he outgrew decades ago - and is protected by Piccolo, just like during his childhood. Demoting Gohan from a mystic fighter who had finally surpassed his father to someone who can barely repeat his very first transformation is a huge middle finger to fans who grew up with the character in Dragon Ball Z, invested in his arc, and would sit in front of the TV after school urging Gohan to release his potential.
Some might argue that becoming a scholar and a father was the natural conclusion to Gohan's story, and that he only ever fought out of dire necessity, not because he was a martial arts nut like the other Saiyans. But there's no reason Gohan couldn't have the best of both worlds - a regular guy who can tap into immense power when the Earth is threatened, rather than a regular guy who gets rusty and can barely throw a decent punch. This is perhaps more of a Western trope, seen in the various Marvel and DC heroes who balance a family life with their evil-fighting duties, but it's one that would've allowed Gohan a happy ending, while not undoing his character development.
As things stand currently in the Dragon Ball franchise, Gohan is not only boring to watch compared to his teenage years, but immensely frustrating, because while Goku and Vegeta continue to grow as characters, Gohan only seems to backpedal, taking beatings from characters his younger self would've dealt with effortlessly. Even after rediscovering a semblance of his former strength in the Tournament of Power arc, Gohan is eliminated after holding down a rabbit for Freiza to defeat, resulting in another anti-climax (this is rectified in the manga when Gohan goes down fighting a fused Legendary Super Saiyan instead).
Despite the scale and speed of Gohan's descent, it's not too late to turn the character around. No one expects him to abandon Videl and Pan to spend years training on another planet - that's his father's gig. But if Gohan could just retain the strength he's already achieved and be a bit more proactive in fighting evil, fans of his Dragon Ball Z days might stop sighing in wistful exasperation every time the character comes on screen.