Marc Cooper of the USC Annenberg School makes his picks:
1. The unconventional election process of 2000
2. The attack on the Twin Towers, September 11, 2001
3. The invasion of Afghanistan, 2001
4. The invasion of Iraq, 2003
5. The introduction of torture techniques as official American policy
6. The unprecedented expansion of executive power
7. Hurricane Katrina
8. The election of Barack Obama
9. The global financial crisis of 2008-2009
10. The media revolution
What do you think? Nominate your own stories in the comments.