What drives players towards harder content?

When discussing Diablo III on Monday, there were some interesting comments from Gevlon, who believed that the AH economy will be driven by people buying items so that they can play through Diablo III at a higher difficulty level. Quote: "However there will be significant social pressure on the player to keep "progressing" on higher difficulties, and don't be a "noob" who just did it in normal." Now I can see how somebody observing MMORPGs could come to such a point of view. The social press

World of Warkraft...