Stifle

For one blue mana, you get to interfere with a layer of the game most counterspells can't touch. A traditional counter waits for a spell on the stack; this catches the ability that goes on the stack after the spell has already resolved or after a permanent is already in play. The fetchland's search, the Planeswalker's loyalty activation, the storm trigger, the cycling draw, the enters-the-battlefield trigger: all of these are activated or triggered abilities, not spells, and all of them are fair game. Two fences keep it honest. Mana abilities can't be targeted, so the fundamental engine of casting things stays safe; and some game actions never use the stack at all, so turning a face-down creature up via morph (a special action, not an ability) sails right past it, even if the trigger that fires once it flips does not. The line that built its reputation, naming a fetchland's activation so it sacrifices itself and finds nothing, is a clean one-for-one in cards that trades a mana for a turn or more of lost development, which is why it lives among the most surgical tempo plays blue has. The discipline is that it does nothing proactively: a reactive blank until your opponent commits to an ability worth the disruption. Reading which trigger is worth the mana, and holding up the answer for it without tipping your hand, is the entire skill of the card.