That was very well said, expatbrit. For one thing, America alone would not have won WWII, so that ignorant argument used at times is just ignorance. But it's a lot easier to argue if you are ignorant of the facts!
But my primary objection to that argument is that it doesn't excuse later wrongdoing by America. America is a great country, but if our leaders do wrong, why is it wrong to point it out? And even if we had been single-handedly responsible for saving the world 50 years ago (and we weren't), it wouldn't mean we could do no wrong now. America does some wonderful, generous things, and they get lots of airtime on our media. America also does some thorougly corrupt, brutal, criminal things, which get almost no airtime on our media, so most Americans remain ignorant of these bad things. It isn't wrong to criticize a country when it does wrong. It is the duty of every citizen to point it out when it happens -- that's part of our constitutional tradition and the very means by which our country was founded in the first place.
Most Americans seem too fat and happy to remember that, and are only too willing to swallow whatever propaganda comes down the pike. 1984, indeed. It didn't only happen in totalitarian countries...