It’s up over 10% now, I believe. They just raised it or are about to.
And if you’re in LA, and they say “20 minutes”, add at least another 20.
And I second the hydration, although warnings of the Southwest go for you other Merkins, too. It may be a dry heat, but it can be a deadly one. Drink up.
I’ve had people being iffy about my passport as age ID in the US before. I thought it was bizarre, “I got into your goddamn country on this thing and you won’t let me buy a drink with it???”
Hell, that drive is exciting compared to I-80 across Nebraska, or driving through the desert Southwest. I grew up in Bettendorf & went to Iowa State, so I’ve made the Davenport/Des Moines drive a number of times, some of them even awake 
Yeah. I don’t know who said it, but to paraphrase: “Only an American would think 200 years is old. Only a Brit would think 200 miles is far.”
An driver’s license for the state you are currently standing in. That’s not the way it should be but as a practical matter the driver’s license is the de facto ID standard in much of the US.
Most Americans do not speak a second language. Unless you are from a Spanish speaking country, you will need to learn, or be able to sound out basic English from your phrasebook. Though it has no official language, nearly all Americans use English; other languages tend to be used within the home or ethnic communities. If you are interacting with a local, do not hold protracted, unrelated side conversations in your native tongue, as many Americans find it uncomfortable or offensive. This creates an atmosphere of exclusion to those who cannot understand you when you are conversing with them.
Many Americans find the chance to learn a few words of your native tongue an enjoyable experience however, so do not be afraid to trade a phrase or two with the local who is helping you.
Telling the local how to say, “Fuck you and the horse you rode in on!” in a new language will forever gain a place in their heart.
They are badly outnumbered by the alligators.
I enjoyed the drive from LA to Las Cruces much more than my drive from Chicago to Lincoln.
Americans, on the whole, don’t travel much and the two countries we most often travel to haven’t required passports until recently so they’re not something that the person working at the local Kum & Go is going to be incredibly familiar with, no more than the cashier at a pub in Dover is going to be terribly familiar with a Wyoming driver’s license.
30 year out of date indeed - for skimmers, please note that 15% is now standard and many if not most tip 20%.
Americans tend to really like foreign people who just come, be polite, and leave. You get major bonus points for being Australian, Irish, British, or some other culture that is vaguely but not much like ours. Scandinavians can be really good too. The odds of getting laid swing very much in your favor if you land on the U.S. mainland with these credentials and your are the least bit attractive.
Strong American patriotism is real but it can easily work in favor of foreigners on vacation. We want to show you how great everything is here even if it means hospitality or maybe even a little roll in the sack to convince you. The service industry tends to be more friendly than other cultures as well. Some people call it fake but it really isn’t because the whole thing is so ingrained. If a waiter or waitress in the Deep South asks about your home town in say, Ireland, they are dead serious and want to listen to what you have to say.
Cheese and rice! My warning: don’t be afraid of anything in the US because of anything you read in this thread! Of course, you did ask for warnings without asking for odds of these unpleasant experiences. Day to day living is not filled with danger over here. Wild bears have killed nine people in the US in the past eight years. So, don’t smear yourself with food and go wandering around in the forest and I think you’ll do fine.
Sharks, lightning strikes, pterodactyl attacks: very, very rare. Talk all you want about politics; everyone you meet will passionately hate any and all politicians, so it’s not like you’re going to cause a ruckus. Even religion; just avoid anyone who’s picketing a funeral carrying a sign that reads, “God hates fags.” Most of the other religious people are just fine.
The big threats (to echo some commentators above): [ul]
[li]If you’re going outside, bring water and sunblock, because we have a lot of outside.[/li][li]Distances are longer; account for longer commutes. In LA, people don’t bat an eye if you say you drove 40 miles to a nice restaurant.[/li][li]Please speak slowly, and be prepared for us to speak slowly.[/li][li]Most stores won’t accept Euros (though they probably should)[/li][li]Don’t listen to any AM talk radio[/li][li]In many urban areas, smokers are basically second class citizens; many rural areas seem to be more tolerant[/li][li]Don’t give money to anyone on the street for any reason[/ul][/li]
IME, people are nicer drivers here (except when they shoot guns which, again, doesn’t happen too often), and will probably be happy to give you directions if you’re lost. (I’m assuming you’d ask directions of the nice woman with two kids holding ice cream cones over the man in the scraggly beard having an argument with a lamppost).
Be ready for long delays at Immigration on you way into the US through the airport. Americans do not realize what a pain formalities can be nowadays. Be prepared to wait, to answer a lot of questions and to be bored.
If you fly right through, which is really most often the case, enjoy the shops in the airport while you wait for your connection.
I looked up some more modern advice before heading to the US, but Dad’s (outdated) advice was still useful – being young and naive at the time – because if you don’t come from a tipping culture then this aspect of American life really does need to be explained… or it will come as a terrible shock. 
(Not wanting to create a tipping hijack, but even now… after visiting the US several times… I’m still somewhat unsure who gets tips. Waiters I get… I’m paying for the food and tipping the wait staff… OK… taxis I know… but I’m paying for the taxi ride aren’t I? And if I tip the hairdresser what am I paying for since a haircut is all service? I’m OK with the custom when in the US – when in Rome and all that – it’s just that not having the custom ingrained makes every financial transaction an opportunity for considerable social unease).
I warn my friends from Korea that the USA is a significantly more dangerous place than what they are probably used to. There are few places I’ve ever felt as safe against criminal harm as Seoul, and I come from a stereotypical small Southern town of low crime rate fame. I wouldn’t walk home alone on the majority of the streets in my hometown, but people regularly pass out drunk on the streets here and don’t get robbed.
Most of the native Koreans I know here are absolutely astounded by the notion of daily missing persons reports.
Which gets me to wondering, why is Seoul (a city of some 14 million people) so much safer than even some of the safest cities in the US? Especially when it seems intuitive that larger cities will have more crime?
Don’t drink the (bottled) water. It’s a ripoff and not healthy. Tap water is best (except in New Orleans and similar environs).
Between Denver and California, gas stations might not come along as often as Euros are used to.
Outside of big cities, and inside some big cities, walking home drunk, grab-assing with your friends might get you in trouble with the cops. Being drunk in public can buy you a night in jail.
This one is my opinion, but, I don’t think most Americans feel as close to their government as many Europeans do. It’s an extreme example, but I’ve had some Icelandic friends gripe to me about George Bush like we were golf buddies or something. I had to point out that this isn’t Iceland were the boss has his/her phone number listed in the book.
Don’t travel by Greyhound, unless you’re really nostalgic for that Third World hellhole you visited last year.
Timepasses, Timepasses, Timepasses.
Do you want to go to the base of the statue of liberty*? Then you need a timepass. Some of the major national attractions in the US require one of these now and you don’t want to fly several thousand miles just to be turned away, right? Look up the attractions beforehand and see if you need to schedule a timepass. Then do it.
Remember, its only a “Free Country” in so much as how many dozen dollars one Euro now buys.
Cite1: from NPS - Page In-Progress