If we were astonished a decade ago when rye whiskey was suddenly rediscovered by young mixologists, the biggest gobsmacker is coming now. The rye rage is no mere splash in a cocktail shaker. More than just a way to inject nuance—and historical integrity—into classic drinks, the whiskey has developed into a connoisseur's category. And signs indicate that the explosion has yet to begin in earnest. Wine Spectator's Jack Bettridge explains.