Here's what NGC says about their "★" designation: "Toned coins can be of a single color or multicolored but cannot have any areas that are dark brown, approaching black." And yet the photo that accompanies this text shows a toned coin with areas that are dark brown, approaching black(!) Yeah, okay... maybe the coin isn't really brown-black, but I just thought the accompanying image was a funny inclusion
Here is one that starred for cameo contrasts... I can't find images of a semi PL coin at the moment, but these are some of the qualities that garner a star.