The culture aspects of the Xiongnu that were immediately noticed and were very different from the Han ones were that they did not lived in cities and villages and were not very interested in architecture and agriculture, but instead they were nomads that were constantly on the move in search of grazing lands and food supplies.
This was very weird and unusual in the eyes of Sima Qian, and it is one of the biggest reason why he (and pretty much all the others in Han China) considered the people from the Xiongnu confederation as primitive and barbaric people.