Badass Decay: This game saw the debut of the Immortal cards, which serve as a set of Infinity +1 Swords. They had powerful and unique effects, even aside from the natural benefits of being Immortal (immune to Crush terrain, and create it upon death, likely killing any opponent who beats them). Unfortunately, when they were released in the actual game, they mostly lost their awesome effects or were just plain not very good — only Slate Warrior and maybe Rigorous Reaver were still sorta playable. How bad was it? One of the Immortals was Shapesnatch. Yes, that Shapesnatch.
Critical Dissonance: The game got mixed reviews, but it still became a Greatest Hits title with over a million copies sold.
Critic-Proof: Likely the reason for the above. It was one of the first Yu-Gi-Oh! video games for home consoles released in the early 2000s at the height of the franchise's popularity — it was going to sell no matter what critics said.
Designated Villain: The Yorkists are supposed to be the bad guys, but if you play both campaigns, it's pretty clear who's offering the better deal. If you side with the Lancasters, Henry VII gets crowned King and takes the credit for winning the war, to the point where you, who did all the work, aren't even mentioned in any recorded history, and it's unclear if you even returned to your own time. Meanwhile, siding with the Yorkists leads to Seto treating you as an equal, even planning to share the the Guardian's blessing with you. When the summoning doesn't work out, he gives you a pendant that guarantees his family's debt to you, and gladly returns you to your own time. Furthermore, we're told that the Yorkists have the Lancasters pushed into a corner, but a quick look at the actual map shows the Lancasters being everywhere and equally strong as, if not stronger than, the Yorkists. On top of that, regardless of what side you choose, Henry becomes king anyway and Seto steps aside.
Darkness Approaches. It flips everything on the field face-down, meaning flip effects can be recycled. A good number of cards power up themselves when they're flipped face-up, such as Wood Remains and Mystical Elf. Not only that, Fiend Reflection #2 lets you play another card the turn it's flipped, and even worse, Magician of Faith and Mask of Darkness can recycle spell cards. Including Darkness Approaches, which then flips them face-down, creating an endless cycle. Admittedly this is a card you need to be very lucky to obtain.
The easiest ones to obtain and work with are Zombies. The person using them is fightable as the second opponent in one campaign, and despite his low deck cost he isn't extremely difficult to beat. The archetype runs on two principles: Multiple copies of Dragon Zombie, Armored Zombie, and Clown Zombie, three monsters with extremely low cost for their attack power and low Level (they exchange this for Defense, which you rarely need), and the ability to summon multiple copies of Pumpking the King of Ghosts (through normally summoning him or through fusing a Zombie and a Plant together, when one of them is at or above 1000 attack). Pumpking by ITSELF is a Game-Breaker, as its ability (raises the ATK and DEF points of all of your Zombies permanently by 100 every player turn) allows for insane ramping of power even with only one on the field. Throw in Wasteland to set the terrain, a few more cheap Zombies, and just a few more cheap cards (like Swordsman from a Foreign Land and Cursebreaker) into the deck and you have an easily obtained deck that can beat 90% of the game without modification. Add in Call of the Haunted and/or Blue-Eyed Silver Zombie to change all your non-zombie monsters into zombies to get pumped. Just watch out for Crush terrain. Or use a Field Spell to remove it.
Perfectly Ultimate Great Moth is pretty nasty. It has the ability to lower your opponent's attack and defense on every creature (even face-down) permanently every turn as long as it stays in face-up Defense Mode. A few duels with Weevil can get you the Cocoon of Evolution and a Petite or Larvae Moth to fuse with it to create Pupae of Moth, which turns into Perfectly Ultimate Great Moth next turn, and goes to the graveyard. Where it can then be resurrected with a card to get you another Moth (Mimicat is commonly used for this). Then it goes to the gravevard again. You can use these cards in your already game-breaking zombie deck. Humiliation doesn't even begin to cover it.
Once you draw Mirror Wall (obtainable with a password at start of the game if you know it), you've practically won the duel. It's an incredibly powerful continuous trap that blocks all of your opponent's attacks and cuts their monsters' attack in half. However, due to Artificial Stupidity, the A.I. will simply keep attacking as if the trap didn't exist, weakening their monsters and allowing you to do whatever you like unimpeded.
Get lucky with some reincarnations, and you'll find Muka Muka. Its cost is a paltry 14, the same as Clown Zombie, and though it starts with 600 ATK, when flipped up, it gains 300 for every single monster in your Graveyard. Play it in the late game, and it'll probably have something in the range of 4800 ATK, and even in the early game, simply discarding your hand is enough to make it fairly strong. Combo it with the above Darkness Approaches combo, and it'll usually have enough ATK to OTK the opponent while attacking their strongest monster. And if you don't like reincarnation, Swordstalker does basically the same thing but better in some ways (2000 ATK, 100 for every monster, meaning it's stronger than Muka if you have less than seven) and can be found through a password or dueling Kaiba or Pegasus.
Gate Deeg and Berformet instantly max out your summoning stars, neither have especially high costs, and the former can be played on the first turn. This makes summoning strong monsters insultingly easy. First turn: Gate Deeg. Second turn: Blue-Eyes. Third turn: Red-Eyes, or one of its fusions if you're lucky. Even if you don't use Darkness Approaches, Gate Deeg basically gives you up to four turns worth of summoning stars, and your opponent won't keep up.
Greenkappa's stats change to match that of the strongest monster on the field when it flips up. Depending on the situation, this either just let you copy your opponent's ace for a possible Taking You with Me at worst, or it gave you two of yours to stomp the opponent twice as hard.
Heartwarming Moments: Going by real-life history, Yugi and Tea will get married after the game. (And for bonus points, most historians believe that Henry VII and Elizabeth of York's betrothal led to a Perfectly Arranged Marriage.) When you duel Tea, she fights for her own sake and to protect the man she loves.
Older Than They Think: Seto and Yugi's battle themes are remixes of tunes from Monster Capsule Breed and Battle, a Japan-only game.
Surprisingly Improved Sequel: Yu-Gi-Oh! Forbidden Memories was, to put it kindly, crap. Strategy was rock-stupid, balance was flatly nonexistent, the difficulty was a brick wall conquered only by monotonous grinding, and the gameplay was profoundly unfun. Its only advantages were its somewhat novel story mode, its animations, and capturing some of the feel of Duelist Kingdom. Duelists of the Roses, filling the Spiritual Successor role, features actual balance, a card pool large enough to pursue any number of unique strategies, a reasonable level of difficulty, and actually fun gameplay, while giving two story campaigns, better animations, and one of the best interpretations of the Duelist Kingdom rules in the franchise.
That One Boss: Pegasus yet again. He's blocked from immediate approach by the labyrinth squares ahead of the player, which gives him time to set up some rather nasty spells and traps such as Change of Heart or Brain Control, though most of the time he'll just spam Infernal Dance and deal 1000 points of damage, baiting you towards him before he snuffs all of your LP from a distance. His deck also makes great use of the surrounding Toon spaces while practically every other monster doesn't. Finally, as mentioned under The All-Seeing A.I., he generally doesn't fall for most of your tricks and bluffs. He is quite possibly the hardest duel in the game aside from perhaps Seto or the final bosses.
His card set is designed to abuse his Millennium Eye too. The fact that he can cast Change of heart and takeover a facedown to hit you or suicide a weak monster against your strong one to force a ton of LP damage is bad enough. The fact that he knows when he can get a direct attack in with Brain Control (takes over your strongest) even when that monster is facedown is unforgivable.
Tier-Induced Scrappy: A number of types get the short end of the stick in this game, just as much as the card game gave them at the time. Dinosaurs are by far the worst, with only thirteen Normal Monsters to their name and nothing stronger than 2200. Reptiles and Pyros aren't much better, with thirteen and eleven monsters, respectively, but they at least have one or two okay cards, even if getting them is an ordeal. Conversely, Zombies, Machines, Insects, and the Fish/Aqua/Sea Serpent trio tend to dominate.