Glass Cannon

Glass Cannon is a  passive ability, unlocked at character level 16.

In-game
Due to diminishing returns, the negative effects of Glass Cannon are less significant as the Wizard's protection rises. It is also a very good ability for leveling up, as before level 60, armor and resistances mean little for survival, especially when in a multiplayer game with a higher level player. It is also a popular addition to Archon, as the loss of armor and resistances is compensated by Archon's bonus.

It is one of the two Wizard skills (the other being Energy Armor), and one of the few in the entire game, that actually decreases one of the player's stats.

Trivia

 * "Glass cannon" is an idiomatic term used to describe deadly and powerful, but vulnerable characters, devices, constructs etc. in fantasy and fiction. In the Diablo III players' community, it has become a general term for ranged builds that focus on the highest possible damage, but with a minimum defense.