Voidslime

Counterspell answers a spell, and for most of the game's history that was the boundary: the stack item you targeted had to be a spell on its way to resolving. This widens the target to the two things that normally slip past hard counters: activated abilities and triggered abilities. That extra breadth is what justifies the slot. A Counterspell does nothing about a fetchland crack, a planeswalker ultimate, an Eternal Witness recursion trigger, or the dies trigger that wins someone the game; this stops all of them, because it cares about the ability sitting on the stack, not the permanent that produced it. The mana-ability exclusion is the one fixed wall: anything that produces mana without targeting bypasses the stack entirely, so you cannot Stifle someone out of their own lands, and a Cabal Coffers tap is untouchable for the same reason. Everything else with an item on the stack is fair game. The price of that flexibility is the casting requirement: double-blue and a single green keep it a Simic-or-bust card rather than a generic splash. What it represents is the merger of two counter philosophies into one card: the blue impulse to deny a spell, and the green-blue (and historically red) tradition of Stifle-style ability denial, welded together so a single piece of interaction covers both halves of the stack. For a control deck that hates surprises, that coverage is rarer than the rate suggests.