Thoughtseize

The two life you pay is the entire conversation. Stripping a card from an opponent's hand before they can use it had been black's province for years, but Duress couldn't touch creatures and Hymn to Tourach asked for two mana and random selection. This collapsed the cost to one mana and the precision to surgical: see everything, take the one card that breaks your plan, and the only tax is two life off your own total. That symmetry is the design's whole point. The discard is free of friction, so the friction lives on your life total, which is exactly the resource an aggressive deck can spend and a slower deck cannot. Against fast clocks, a turn-one Thoughtseize can be the most expensive two life you ever pay; against a grindy control mirror, it is nearly free. The card doesn't grade itself by matchup, it makes you grade it, every single cast. That is why it has stayed legal in formats that ban far less efficient effects: the self-damage is a real governor, not a token one. Discard this precise and this cheap should warp a format, and the life payment is the reason it doesn't quite. Two decades of designers have circled the same problem (how to print proactive, information-rich hand disruption without making the play free) and the answer keeps coming back to some version of this trade: you get to see the hand and take the right card, but you bleed for the privilege.