Another reason for Anti-Americanism could be that the United States is held to a higher standard, because US presidents make bigger promises than any European head of state or government.
The mismatch between by the US rhetoric and policies invites harsh criticism. Take for instance this promise from President Bush's second inauguration speech in January 2005:
Today, America speaks anew to the peoples of the world: All who live in tyranny and hopelessness can know: the United States will not ignore your oppression, or excuse your oppressors. When you stand for your liberty, we will stand with you.
And then look at the US response to the uprising in Burma in autumn 2007. The Bush administration did not stand with the monks. The European governments were not any better, but they did not make big promises about supporting liberty.
Burma is pretty much vanished from the European and American mainstream media coverage. Oh, wait a second, AP just reports that "US First Lady Laura Bush has conferred an award honoring the women of military-ruled Myanmar in their determined battle to restore democracy and human rights in the Southeast Asian state." Awesome. The US has once again shown great leadership. I am sure this award will end tyranny nd bring liberty to Burma.