There is no objective way to measure whether something in a game is fun or not. But in the context of competitive online gaming, where empowerment for one player sometimes translates into abject misery for another, it's much more common to encounter a mechanic so demoralising, so soul-crushingly boring, that you simply know in your heart that it is bad and should be done away with.

Right now, in Hearthstone, that mechanic is Taunt.