You as an individual American are probably quite lovely. One of my dearest friends is in Seattle, and she's lovely despite the disadvantages of her background.
However, just as a stereotypical bunch of Brit's can get up the noses of people from other counties, by complaining about the tea, being vaugely smug, acting like Britain is still a World Power, laughing for inexplicable reasons, having a strange sense of humour, and having legs so white from lack of sun they tend to be slightly blue, a group of Americans displays equal but different annoying stereotypical qualities.
These include a depressingly low level of knowledge about anywhere else in the world coupled with a conviction that America is the best country in the world, moaning about bad dental work of other countires when their own country has no real equivalent of Public Health Care, never-ending complaints about bad service and bad food when they come from the country that bought us McDonalds, cancelling visits due to terrorists scares when they have a greater chance of being shot in their own city, ghastly taste in clothing in retirement, a habit of 'ticky boxy' tourism ("Europe in only nine days"), and a very poor grasp of higher forms of humour such as irony and sarcasm. That's without going in to the butchering of a language on a daily and systemised basis, or thinking that American Football is a proper sport, or voting for George Bush Junior. You also as a group tend to moan about how silly it is to have a monarchy, but tend to be bigger fans of the English monarchy than many British people.
You see, you're all individually lovely, but as a country you suck. However, American's generally don't think their country sucks as, although they are lovely, they are rather clueless.
I think most of the Brits will be quite happy to agree that, in many ways, Britain does indeed suck. We spend a lot of time talking about exactly how much Britain sucks in homes and pubs, restaurants and workplaces.
It's this habit of taking themselves seriously that really rankles many British people. Of course, the British trick is to pretend we take ourselves far less seriously than we actually do. It comes in useful, especially in sport and cricket (notice the AND). But I don't think it would be possible for Britain to have a clause excluding politics and religion AND open Congress with prayer AND have 'In God we trust' on our money, as that's so obviously silly.
Obviously these are gross stereotypes. American's characterisation as a nation of TV zombies contrasts with the fantastic libraries, gallaries and museums I have been to in the 'States, and your bookshops are wonderful. I love the customer-oriontated culture.
But you American's shouldn't take the fact we get regular amusement from your sweet little colonial antics and gapes personally. We are not laughing at you individually...