Phil Maymin asks:
What one ingredient most signals a quality dish to you? For me it is scallions. If it's got scallions in it, it's gotta be good. Scallions have never steered me wrong. I think it's because no one really starts with scallions. They get added later to take a proven dish from good to great.
His answer is excellent. "Fish sauce" came first to my mind, perhaps because it doesn't taste so good. Yet it is correlated with high-quality Thai, Vietnamese, and other creations. Real saffron (expensive, signals quality) and lime (sour, scares off the timid) also could be mentioned. Chicken gizzards. Sichuan peppercorns. What else?