In the USA, to be un-American is to be intelligent, rational, not wishing to own a deadly weapon, tolerant of other cultures and religions, not rationalising everything on "well Jesus said this", supportive of fair taxes, against needless wars, having a healthy lifestyle, not be overly loud and brash, and generally a nice person. An example of this person is Barack Obama
This is the exact opposite of what it is to be an "American", for example the other 99% of the population.
In Europe, to be un-American is to be the type of tourist we would like to visit and the type of country we would like as an ally.
American person: "Look at that liberal, he wants fair taxes, people not to have guns and he doesnt even go to church every sunday, he's so un-American"