GREAT PLAYS are usually great in one of two ways. They are either culminating examples of existing ideas or groundbreaking examples of new things entirely. The Humans, by Stephen Karam, at first seems like it will be one of the former. Taking place at Thanksgiving, in the large but creepy Chinatown apartment that Brigid Blake, 26, and her boyfriend Richard Saad, 38, have just moved into, it situates itself squarely in the theatrical tradition of family-at-the-holidays plays. Richard is wealthy and apparently easygoing, but the Blakes, from working-class Scranton, are under every sort of stress imaginable: marital, socioeconomic, romantic, cultural, medical, existential. The parents, Erik and Deirdre, have discomfiting news they keep hoping not to share; Aimee, Brigid's older sister, mostly maintains the brave face of someone who, having been…