Here is a list of performances where the US didn't perform to expectations:
1) Men's 4X100 relay
2) Women's 4X100 relay
3) Bernard Legat fails to qualify for 1500m final
4) Lolo Jones hits 9th hurdle and finishes 7th
5) No medals in the Women's 100
6) Sanya Richards poor performance in the Women's 400m, finishing with bronze
7) No men in the finals of the high jump or long jump
Blah...