The Scene: At their mother's behest, siblings Johnny and Barbara are on their annual sojourn into the Pennsylvania countryside to plant flowers on their father's grave. They see some shambling form moving in the distance - a drunk perhaps, or just some cemetery caretaker. Johnny teases his sister, warning in a ghoulish voice that 'they' were coming to get Barbara. As Johnny plays upon Barbara's fears in the isolated and gloomy field, the stumbling man attacks them, killing Johnny. In a panicked frenzy, Barbara narrowly escapes the madman and makes her way to a lone farmhouse. There she, along with a handful of strangers, is forced to stave off the throngs of the undead.
Why It's Iconic: Johnny's moment of silliness is the last respite before the siege. For a few moments, it's nothing but a brother tormenting his sister, but his words, "They're coming to get you, Barbara," become horribly prophetic. They were indeed coming to get Barbara, but even Johnny couldn't know what that would mean. His skull being smashed on the tombstone signals that the film is about to turn a corner. In a decade mired in campy alien attacks and irradiated, giant insects, the movie took the rest of the genre with it, into a socially scathing, nihilistic territory. It represents everything that came before that moment - cheap monsters in rubber suits devouring the initial characters on their way to the prom. From that moment director George Romero redefined not only zombies, but the whole of horror.
Imitators/Flatterers: The references to this scene in subsequent films are countless. Most of them take the approach of not the foreboding warning, but one of camp. Oddly, the quote has come to represent everything that it isn't. Without the context of the film, the quote and scene are cliched. It's Boris Karloff, lurking in the shadows of a crumbling castle. Still, those filmmakers 'in the know' use it with reverence, most recently (and notably), in Shaun of the Dead.