On top of being the first series to not have its protagonist interact with other Yu-Gi-Oh
The Normal Monster "Harpie Lady" and its first four Effect Monster retrains ("Cyber Harpie Lady", "Harpie Lady 1", "Harpie Lady 2", and "Harpie Lady 3") are considered as the same card when constructing a Deck: a player cannot use three copies of the original "Harpie Lady" and three of "Cyber Harpie Lady"
You can even build around Alms Collector to an extent, running cards like Secret Rendezvous and Your Temple is Under Attack to deliberately give your opponents cards
Cloudstone Curio Cloudstone Curio goes infinite with a ham sandwich
If this card is flipped face-up: During the End Phase of this turn, change as many face-up monsters on the field as possible to face-down Defense Position, then you can Special Summon 1 "Ghostrick" monster from your Deck in face-down Defense Position, whose Level is less than or equal to the number of monsters flipped face-down by this effect
Whenever a creature you control deals combat damage to a player, goad each creature that player controls