Because of the economic boom during the song Dynasty, more and more peasants moved to cities, which presented a decided social change. During the Song Dynasty people of all social levels accommodated changes in lifestyle, such as switching from drinking wine to tea and from eating wheat and millet to rice.
I think #3 is right in focusing on the role of innovation and discovery. The discoveries listed and the developments in culture through drama and dance certainly made this period of history something of a golden age for those lucky enough to be able to participate in it. What was notable about this dynasty, however, as #2 points out, was that various laws made the enjoyment of such factors more spread out amongst all sectors of society.
Well, for one thing, printing and commerce spread through the country, ushering in a new middle class of people. Gunpowder was invented, people could learn to read and write, and expression through the arts became more common. This was a wonderful era in which to be living and all classes of people benefited from it!
There are so many ways in which this happened. For one thing, changes in laws about landholding as well as in taxes created a system where there were many more private landholders who had small and medium amounts of land. This led to a more egalitarian society.