Abrupt Decay

The mana-value clause is the entire personality. Two cheap mana for unconditional destruction of any nonland permanent would be obviously broken, so the design caps the target at mana value 3 or less: it cannot answer the planeswalker that just ultimated or the seven-drop bomb, but it kills nearly every cheap creature, every two-mana artifact, every cheap enchantment, and the bulk of the things a fast deck actually deploys. That single restriction is what lets it carry the second clause, and the second clause is the rarer prize. Removal that can't be countered is unusual at any rate, and pairing it with instant speed means it slips under the control deck's open mana to resolve before a counterspell can answer it. The combination defined a particular kind of golgari toolbox answer: a card that trades cleanly into the curve of an aggressive board while remaining live against the permanent-based engines a midrange opponent leans on, without ever caring whether the opponent is holding a Counterspell. The tension it resolves is the classic one for any catch-all kill spell: how do you make a two-mana destroy-anything effect without making it strictly better than every conditional removal spell already in the format? The answer here was to draw a hard line at the top of the curve and gild everything below it.