You arein for a big surprise.
There are tremendous cultural differences between the US and the UK, despite having many similarities. People talk different, and not just in the accent or the words, but the general tone and meaning. They have different ways of making friends, finding jobs, interacting at the office. The social system is vastly different from what you know, and religious differences maycome as a bit of a shock.
And when all this comes together as part of a big move, you can find yourself wondering 'what have I done???'
In some ways, try to envision being an expat in rural Mongolia, with a different language, culture, climate, time zone--in essence, nearly everything vastly changed from what you accustom to. You go in knowing it is going to be an adventure--there will bethings that don't make sense and things that seem strange. Approach your trip to the United States in a similar way, something radically different from what you expect, and you may find the transition goes a little more smoothly than those who think it is just a simple 'hop across a big lake'.
Now, as for some specifics...you could write a book, or several, on some of the differences. Withouttrying to get into stereotypes, a few observations:
1) Americans are outwardly very friendly, they will talk to strangers in a store, laugh with someone at a bar, and help their neighbours in a pinch. That said, many ex-pats have reported difficulty in making 'really close friends' like they had back in the UK. The friendliness is only on the surface, so many report.
2) Americans generallydo not like / 'get' satire. Some ex-pats claim "they just don't understand it" while many Americans think it "is just a waste of time by those who have nothing substantive to say." Satirical comments in the workplace are almost always misunderstood, and generally not done in social settings either.
3) Many Americans are very family-oriented (at least those with families). Family life is anendless parade of school musicals, extracurricular sports, birthday parties and the like. Many new ex-pats report exhaustion trying to keep up with all the things to do with the kids when they come to the states.
4) The majority of Americans have never been abroad. They may have heard of Spain but have no clue about Ibiza for example. American media does not speak much about Europe, let alonethe rest of the world, and the issues facing 'Brussels' and the 'EU' are of absolutely no interest whatsoever to 99.999% of Americans (in fact, polls indicated that a majority has negative feelings toward the UN). On the converse, the United States is a huge country. You could fit most of Europe in only 1/2 of the area of the United States. If you want a more direct comparison--the entire land massof the United Kingdom could fit within the state of Michigan. Go look at a map of the US to get an idea. Many Americans feel they simply haven't seen enough of their own country to warrant travelling abroad to see another country. Just as Americans are made fun of for not seeing the world, many ex-pats are ridiculed for seeing only New York and Florida or other large 'tourist' cities withoutseeing the vast expanses in between.
To add, many people just don't get enough holiday time to make it financially worthwhile travelling abroad, with holiday - "vacation" time often being as low as 7-10 days a year, and in some instances that includes sick days, the norm being that you have worked for a year before really getting any holiday entitlement...because of that, you'll see a live to work...