Yu-Gi-Oh! has gotten more critical censorship than almost other anime that made its way overseas. Completely redirected and rebranded by 4Kids Entertainment as a kids' show about merchandisable trading cards, the watered-down content American television watchers were exposed to basically made Yu-Gi-Oh! an entirely different anime. With Yu-Gi-Oh! censored in America, like Sailor Moon before it, every ounce of adult content, blood, breasts, and even entire seasons were deemed inappropriate.
Unfortunately, these compelling elements had to be abandoned to make the series a more wholesome experience for Western children. Poor Yugi and friends never stood a chance as soon as executives figured out that making trading cards is basically like printing money. Yu-Gi-Oh! eventually made it to the long list of anime censored in the US, leaving audiences with a Frankensteined hybrid of psychologically horrifying, occult Japanese anime and children's daytime TV.
Dark Magician Girl's Breasts Were Made Smaller
In the original Japanese run of Yu-Gi-Oh!, Dark Magician Girl was made purely for fan service. Her character is spunky, master-apprentice relationship fetish fodder with a short skirt and large bust, but 4Kids Entertainment had different ideas. Any hints at cleavage were removed from the character's card (and the holographic representation in the anime) by blurring out the chest lines, making her appear much smaller and more childish.
The Shadow Realm Is Totally Made Up
The whole concept of the "Shadow Realm" is a fabrication of the 4Kids localization team. Instead of characters dying when their mind is crushed after a game, in the English dub they're banished to the Shadow Realm where their souls are tormented forever (but their bodies are totally okay, somehow). The concept was developed from a line by Maximillion Pegasus, who used the "Dark Dimension" to describe the prison where souls became trapped.
Guns Were Edited Out To Look Like Pointing Fingers
Instead of completely editing guns out of the anime dub, 4Kids "cleverly" replaced guns with other contraptions, like when Solomon Muto was knocked off balance by a slingshot, instead of a gunshot. More ridiculously, on multiple occasions, characters just straight-up use finger guns. Instead of completely getting rid of the weapon in hand or editing out the scene, characters simply point their fingers like a gun while their hostages accept their fate. The subtext, presumably, is that they're worried about wet willies.
There Was A Whole, Dark Season Without Card Games
There's an entire season before the Duelist Kingdom arc, titled Season 0, that never made it to the eyes of American TV audiences. This season involved a much darker version of Yugi. It still contained Shadow Games, but they weren't based on trading cards. When Yami Yugi possessed Yugi, he'd challenge criminals and bullies to lethal games which led to gruesome falls and bodily combustion. To easily integrate the card game into the series, 4Kids just skipped over this season and didn't include any games without Duel Monsters cards.