To the Americans who mentioned they were upset by what's been said on here, I think it's important to filter out the unpleasantness and actually, a lot of what's been said is either positive or just objective differences. The type of people who say things like 'loud, rude, obnoxious'... etc are generally the type of people who haven't visited the US (or many other countries) and so their observations are based on lazy stereotypes and what they want to be true to make themselves feel superior. Most people haven't said this and don't feel this
way though!
I lived in the US for several years. I found Americans normal-volumed (!), genuinely friendly, positive and welcoming. I think Brits really are more negative and I noticed it a lot when I moved back. Listening to my parents/grandparents when they get on their soapbox makes me lose the will to live 
I can also hear a Brit from a mile away when abroad, whether in the US, France, Spain etc and I think it comes from hearing something out of the norm.
I was taken a bit aback by Americans' directness when I first moved there - my 2 year old was having a tantrum as e'd just arrived and he was jetlagged and tired and I was told by the person serving me that I needed to get a handle on his behaviour before he got much older. But equally, i think they find our skirting around saying what we actually mean frustrating.
I do categorically not think that Brits are more polite, I think this a myth we like to perpetuate! Yes we say please and thank you more, but there is more to politeness than that. American teenagers we're far more polite than most of the British teenagers I have met. Someone on another thread said there are no rude nations, just different norms and expectations of what politeness is and I think that's right.