Already mentioned in another post by me, but when in the U.K. never refer to a belt bag as a "fanny pack" or you'll get odd looks.
And when in the U.S., refer to the red thing at the end of a pencil as an "eraser", not a "rubber". Additionally, the "S" in "eraser" is pronounced as an "S", not a "Z" - and as most know, this letter is called a "Zee" rather then "Zed", and so it goes on.
In England, after a few pints you might get pissed, but in the U.S. that (being pissed) usually happens when someone annoys you. You have to add "off" to achieve the same meaning in England. On the other hand, "off" never needs the additional preposition "of" in front of whatever thing you are coming off or subtracting from.
I think I learnt ("learned" in the U.S.) most of this stuff in the first year of my expatriation.
Way too much to fit in one post. Chris has a pretty comprehensive coverage in the main area of The Septic's Companion, but you really have to live in both cultures a while to get the best grasp of all the major differences and subtle nuances.
And of course each state has its own variations, just as different areas of Britain do; leaving aside whole languages such as Welsh, which I believe Septic Tone is quite happy to do in any case