Concordant Crossroads

The World supertype is what makes this single line of text one of the most studied one-drops in the game's history. World restricts the battlefield to one copy across all players: cast a second Concordant Crossroads and the older one falls off, which means the card was built as a symmetrical effect with a self-replacing clause rather than a stacking permanent. That symmetry is also a fiction in practice. The player who untaps next, or the player who just emptied their hand of creatures, is the only one who cares that everything has haste. Elves chains, Hermit Druid piles, and any deck that wants to convert a single turn of mana into lethal damage all treat "all creatures have haste" as the missing piece of an engine, not as a Howling Mine style wash for the table. Green at one mana for a global haste anthem is a rate that has never been reprinted at the same cost, and the line reads now as a combo enabler far more than the curiosity it was first framed as. Fervor and its descendants ask three or four mana for the asymmetric version; this asks one, accepts the symmetry, and trusts that whoever cast it built the deck that needed it.