Certain cards have anime-only special abilities that affect the Field (e.g. "Umi" hides underwater monsters from being seen by the opponent; if attacked, "Full Moon" will increase "Umi's" tide while shrinking amount of land.
Fusion Monsters didn't seem to exist as separate cards that the player has within his/her Main Deck ("Flame Swordsman" was actually a Normal Monster in the second series anime, with the dub showing it as a Fusion Monster, because it was Fusion Monster in the TCG). Some Fusion Monsters were capable of being treated as separate monsters for some card effects and attacks, such as "Blue-Eyes Ultimate Dragon".
Certain Field Types gave certain monsters bonuses, like the Ocean field for WATER monsters which increased those monsters' ATK and DEF by 30 percent. (This was eventually turned into a real card, Umi, but with a weaker effect.)
A Normal Spell Card can be activated during the Battle Phase if it is Set face-down.
A Ritual Spell Card did not need to have its corresponding Ritual Monster in the hand in order for it to be activated.
When a player won with "Exodia the Forbidden One", a player laid out all 5 cards on the field for the computer system to recognize it as a victory condition.
Some cards had alternate names. "Monster Reborn" was referred to as Reborn the Monster for half the arc, and "Yellow Luster Shield" was called Chaos Shield. Additionally, some cards were different types. "Call of the Haunted" and "Negate Attack" were both Spell Cards (with the dub displaying them as Trap Cards to avoid confusion for those playing the TCG).
Only one monster was allowed to declare an attack per player per turn.
When a monster was destroyed by a card effect, the controller of that card would receive damage equal to half of that monster's ATK points. This is especially noticeable when a monster is destroyed by "Mirror Force".
When an attack was redirected by a card like "Mirror Force", it was treated as an opponent's attack.
A monster that is treated as an Equip Card was still treated as a monster and therefore could be used for instances such as Tributing for Ritual Summons.
A player could summon a monster in face-up Defense Position or flip a Set monster into face-up Defense Position.
If a player failed to summon a monster during their turn when they didn't control one, they would automatically lose the duel.
Only 48 hours is given to obtain 10 star chips.
Participation is by invitation only. Each participant is given 5 Invitation Cards, 2 Star Chips, and 1 Dueling Gauntlet.
Participants in the preliminary round must be in possession of at least 1 Star Chip.
Each Duel, a player must wager a minimum of 1 Star Chip.
Players who have lost all of their Star Chips are sent home.
The first 4 players to obtain 10 Star Chips would be admitted entry into the Castle in order to participate in the Duelist Kingdom Finals.
The Duelist Kingdom Finals are set up in a playoff-type mini-tournament.