Metro: Last Light is a shooter. But it's nothing like a Call of Duty, and developers 4A Games resent that comparison.
Metro: Last Light is much more about an Eastern perspective on a post-apocalyptic, everything-went-and-got-crazy world. Mutants large and small still terrorize Moscow, as they did in the first Metro title. And I imagine they're pretty pissed that—Metro 2033 spoilers—you nuked them in the previous game.