Mass Hysteria

The one-line clause every combo deck reads first. Granting haste to all creatures for a single red mana reads like a fringe enabler until you remember that combos built on creatures (Kiki-Jiki loops, free attackers, sacrifice engines that want a body to swing the turn it lands) are all gated by summoning sickness, and this strips that gate cheaper than anything else does. The catch is the word "all": your opponent's creatures get haste too, which makes this a liability in any deck that wins through the red zone and a non-issue in any deck that wins before the opponent's board matters. That asymmetry-in-symmetry is the whole design. Earlier haste-granters were attached to bodies or auras and cost real mana to deploy; pricing the effect at one mana with no creature attached and no off-switch means the card can only be a one-card half of a two-card kill, never a standalone aggressive tool. It is built for the deck that wants exactly one thing (its creatures acting immediately) and does not care what the opponent's untapped board does, because by the time it matters the game is already over.