I didn't exactly mean superficial style, but properties like creativity, liveliness, and consistency. I hesitate to say that these are things LLMs can't do, because I'll provoke somebody into telling me that they totally can, but weaknesses like this could be ways for human intuition to detect an LLM. It might have to be a long competition, because an unusual set of prompts (or training, even) is likely to initially impress the humans: time should be allowed for novelty to wear off.