Snow
Snow is good, in moderation. Falling gently on the trees and covering the ground in a white blanket, it transforms dull winter days into an unexplored wonderland. It's something new and fresh, even celebrated by some.
But too much snow brings chaos. Iced roads form and act as hazards, high snow banks make it difficult to get anywhere, and the temperature can drop low enough to kill. The wonderland is suddenly a nightmare.
Elphaba thinks it's strange that something so delightful on the outset can turn so wicked in large amounts, and supposes that good deeds work the same way.