Disclaimer: I am an American...
Impolite: This mainly seems to come from Europeans and people who have had a bad experience with an American. In general we are louder than them, which really isn't that bad, just different. But when we go on subways or in resturant and talk (often excitedly; its rare that we get to go to a country overseas) it tends to be louder than what it considered normal by the locals and comes across as impolite.
Other cultral differences and the fact that Americans are unaware of them lead to instances where we do something that is completely normal in our own country (such as leaving a resturaunt not long after we arrived) is offensive in anouther country (Germans consider it to be an insult if you don't stay at a resturaunt for awhile).
Fat: Yes, we do have an obesity problem. It is worse in certian states, such as the ones in the South, West Virginia, and Michigan, than others like Colorado. Plus 'we' created Mc Donalds and fast food and use to have the highest obesity rate. Other countries are either catching up to us and I think Austrailia might have passed us, but we were the ones to be the worst first and that's what sticks with people.
Stupid: George W. Bush. He made stupid decisions (bad for our country and other countries, but not necessarily bad for himself and his cronies) and couldn't talk in a straight line (which became worse as his presidency progressed, which makes me think he really wasn't cut out for his job). I personally think of him as Dick Chenney's puppet.
We as a people also tend to not know much about or care to know about other countries(people tend to only bother to know things that directly apply to them, and other countries often don't fall under that catagory), which offends people of said countries(since obviously people from every other county in the world knows a good chunk of things about their country). Other countries aren't featured much in our media, and geographically we only share borders with Canada and Mexico.
Form what I have seen/heard, we export much of our (horrible) media, which features idiodic Americans acting like idiots (reality TV and certian TV shows).
Our education systems aren't doing to well and are getting worse, as they are moving towards a rote system and away from a creativity based system (which is the opposite of what China is doing now...). Whether or not you beleive tests are an accurate measure of intelligence we also don't preform well on them, which only reinforces the stereotype.
Side Note: Americans don't like each other. Northerners and Southerners dislike/hate each other (American Civil War...), although Northerners seem to be more vocal. Liberals and conservatives, Democrats and Republicans loathe each other. Many people don't like the major cities or where they live for various reasons. And so on. This makes it easier for people here agree with the stereotypes.
/end pointless rant.
