Simply put, it's the fact that fans of the series view the characters as treating a simple "children's card game" as a huge piece of srs bsnss. (Thanks a lot for that observation, LittleKuriboh. You have officially warped the fandom.)
Well, it's certainly true that they put an awful lot of stock into a game, but it goes beyond being a children's card game.
You see, when Takahashi was writing the early chapters, there was only one card game that was well-known enough for him to be talking about.
He called the game in his manga Magic & Wizards, but he was really talking about Magic: The Gathering.
(Note Grandpa's emphasis that it was the card game that "is such a hit in America." If you ever get pestered by people talking about the purity of the card game in Japanese due to its origins, there's a stick to hit them with. Yeah, the game is different-but Takahashi thought of it as an American game.)You might dismiss this as an unfounded theory, but aside from the obvious fact that Magic was and is the first, biggest, and most-played card game in the entire world, the cards that Takahashi designed are essentially a simplified version of the M:TG cards.
Not necessarily the individual cards (though I'll get to that). If you look at the card back of a manga or anime Yu-Gi-Oh! card, you'll see that it has a dark brown oval in the center, with a brown "background" and a tan border.
Check out this article on the M:TG site, which showcases the game's card back (with a hypothetical variant back that never was). The proportions are rather different, but it's essentially what Takahashi was building from.
The fronts have some similarities, too; when Takahashi designed his cards, he put stars on them, possibly because he didn't know quite enough about M:TG to know just what the mana cost on a Magic card meant. (By the way, if you ever wondered about the "spell card" change in Yu-Gi-Oh! because you weren't following the game, it's all Hasbro's [M:TG's proprietary company] fault. They thought the use of "magic" was a misleading use of their trademark, as Magic players refer to their cards in a similar fashion. Never mind that Hasbro can't actually trademark the word "magic...")
It does, in fact, go further.
Check out this old picture of the Blue Eyes White Dragon, and then compare it with this picture of Shivan Dragon, one of M:TG's old famous cards.
Note the general similarity of the poses.There's another one that has similarities to an old school Magic card, Summoned Skull. Compare this manga image to Lord of the Pit:
This is further highlighted by Takahashi's anniversary art version of Summoned Skull, which in homage to its origins has the same color scheme as Lord of the Pit, though its appearance has diverged along different directions from the "traditional" Summoned Skull design most of us know from the anime.(Note that, in the two games, their positions of power relative to each other are essentially reversed. Lord of the Pit is [mostly] stronger than Shivan Dragon but less practical, while the same is true of Blue Eyes White Dragon compared to Summoned Skull. Not that any of the cards are competitive in the modern games...)
Of course, any discussion of the early manga version of Duel Monsters/Magic & Wizards would be highly incomplete without looking at Seto Kaiba, everyone's favorite psycho player.
Takahashi has apparently stated that Kaiba was based on somebody who was a huge jerk to one of his friends originally, but the character has obviously moved away from his humble, cruel-guy origins.
I have a theory as to why the character evolved the way he did, and I'm going to share it here. It's pretty crazy, and it hinges on a completely insane coincidence, but I'm sticking with it.
Flash back to 1999. This was during the early days of the anime, more or less. Guess who was rocking the Magic: The Gathering world at that time?
His name was Kai Budde.
Now, Kai Budde is not Japanese (although Japanese players have traditionally had a pretty big impact on the game). He's German. But if you parse his name, well... It's not much of a stretch at all to notice the similarities between Kai-ba and Kai-Budde.
They even look a bit similar, as far as such things go.
(Pictures ganked from the Yu-Gi-Oh! wiki's Kaiba page and Kai Budde's previously linked Hall of Fame bio page. Sorry if it bothers anybody, but I'm doing a historical essay here, and I think it's fair use.)It's impossible that Takahashi derived Kaiba's name from Kai Budde's-Budde's first victories that propelled him to worldwide Magic fame occurred in 1998, and even presuming an extremely leisurely publishing schedule, "The Cards with Teeth" would have come out well before that, sometime in late 1996 or early 1997. (The Yu-Gi-Oh! wiki, being heavily focused on the card game, doesn't have a date for the chapter itself, and I'm hesitant to guess too closely.)
Anyway, why is it important that Yu-Gi-Oh!'s card game is based on Magic?
Well, according to market research, the biggest demographic of Magic players is in the 15-16 year old age bracket. In effect? The same age as the characters from the series.
Further, it might seem silly, but Magic really is serious business. Aside from the fact that certain old cards from the game are extremely pricy (some go for somewhere in the neighborhood of a thousand dollars apiece, which makes participating in a format where those cards are legal very difficult for newcomers), you can actually make big money playing Magic. People do.
There are hundred upon hundreds of players worldwide who compete in sanctioned tournaments, and many of these sanctioned tournaments have large monetary prizes. Kai Budde actually made something of a living off of the game at the height of his career, as I understand it.
Basically, the game we see in the various Yu-Gi-Oh! series is what Magic would be like if it reached the scale of a big-name pro sport.
Which isn't entirely impossible, you know. Just not likely.
Yet.
