Security. We were reminded of the sensitive nature of Chinese possession of Tibet before the visit, which was made only more sensitive by the approaching Party Congress. The photos don't show this, but the plaza of the Johkang Temple was patrolled by police, soldiers, and fire fighters in flak jackets, and all of the market streets surrounding it had police checkpoints controlling access.
The plaza below Potala Palace, with the "Tibet Peaceful Liberation Monument," has all sorts of propaganda images on walls and fences, highlighting the beauties and benefits of the Chinese nation. They clearly were political, and perhaps not particularly clever, since none of the people depicted looked like Tibetans.
Security was tighter at this airport than any others on the trip, with an explosive swab at the door to the departures hall. A squad of soldiers marched through as we were waiting to check our bags.