Library of Alexandria

The condition is the design. Seven cards in hand is the cleanest possible expression of "you are not pressured, and you are not flooded," and the card pays you to maintain that equilibrium every turn for free. The mana ability is a fig leaf; the draw is the engine, and the engine compounds. Every untap step where you can hit seven, you are drawing a card off a land, which means you are pulling ahead on cards without spending mana, without exposing a permanent to removal, and without giving the opponent a window to interact. Restriction on the Reserved List, restricted in Vintage almost immediately, banned in every other competitive format it ever touched: the card's history is a running argument that a free card-a-turn is the most dangerous rate in the game, no matter how passive the surface looks. What makes it a singular design rather than just a broken one is the symmetry of the cost. The seven-card clause is self-correcting in theory (the more you draw, the harder it is to stay at seven) and trivially gameable in practice (cycle, discard outlets, instant-speed plays on the opponent's end step). It is the prototype for a whole lineage of "do nothing, draw cards" lands the game has spent thirty years refusing to reprint at anything close to the original rate.