The stereotypes of a difference in humour between U.S and U.K as Bill points out is absolute bullshit. The other difference i would point out is in language, hence UK shows get bought by U.S channels and re-filmed such as The Office which had as much success as it's UK version. Ricky Gervaise picked up some nice transatlantic awards to match his U.S ones.
The one that really pisses me off is Americans don't get irony. That is wholesale bollocks. I live half the year in the U.S, i know what i am talking about, and i live on irony, sarcasm and understatement. It's a smart thing. The same percentage of U.S people are as thick as shit as the percentage in the U.K give or take the deep south. The U.S simply has more people.
There are some huge cultural differences between the UK and U.S (Say the word "cunt" in front of twenty random Americans then 20 random Brits and watch the reaction) such as manners, friendlyness to complete strangers, food (Try getting a good cheese in the U.S or a decent cup of tea. Man... i bought an English Breakfast Tea in a highbrow coffee shop in Orange County and took it straight back to patiently explain we don't drink the dishwater). A boot is a trunk, a hood is a bonnet and don't ever EVER for fucks sake step outside to "Smoke a fag". You will either be arrested or beaten up...
Unless you are in San Fransisco where you will be applauded until you fired one up... then you will be socially ostracized. I use the Zed deliberately rather than a Zee just to confuse matters.
We think Starbucks has taken over the UK... the reason people have a fence around their garden (yard) in the U.S is so they don't come home to find a fresh Starbucks next to the shed and a puzzled looking dog.
The U.S is famous for it's expensive white teeth, wheras we don't pay for ours. The majority of Americans may as well leave out the coffee and just have hot water, flavoured creamer and 10 sugars in a cup. Americans just do not use kettles, however a lack of a waste disposal unit in a house will just not add up. I am also always mildly suprised that when i buy a fresh banana here that it doesn't have a light coating of blueberry sugar. They drive on the wrong side of the road, yet the roads are in fact big enough for cars, lots of cars... and the roads are not complicated by bends. You can in fact get to wherever you are going by purely turning right in a city. In California you can turn left on a RED LIGHT. Imagine doing THAT for the first time with a cop behind you. That cop incidentally has a GUN and has no doubt aimed it at someone in his carreer no matter how short that is. In the boot (trunk) he also has an assault rifle and a shotgun. You should feel safe, but you have probably been driven past by a civilian equally well armed, just not so well trained.
In a few states, most notably Texas almost everybody has a gun and insects either suck your blood or kill you. Every state has animals that will simply kill you. The most dangerous animal you will come across in the wild in the UK is a slightly grumpy squirrel (And if you can tell me where that quote comes from you get 3 months free membership of my site).
In the U.S just as many kids ride skateboards, except they tend to be good at it. The U.S bitches about the price of gas, yet the advertisements on TV will have you beieve that 35 MPG is a good rate. Watch the disbelief when you tell Americans a lot of European cars run on diesel and outperform American cars in every aspect. Tell them a good European or Japanese car in the UK will go 45 - 50+ MPG and watch them think you are a liar. Then empathise that American cars share the same relationship in a spiral of shiteness and decline as English cars. Except English car companies go bust but CEO's of American car companies fly by jet to Washington to ask for Billion dollar bail outs. Watch America as it uses "The Left" as an insult, then turns to socialism as it bails out industry after industry as capitalism fails horrendously through greed. The same sin that is and always will be the downfall of communism. In the UK a right winger is a conservative, in the U.S a right wing conservative is someone who bends words of the Bible to mean and prove whatever socially absurd agenda they may have. In the wrong part of the U.S being an atheist will get you killed... however for the most part you will be defended saying whatever the fuck you want to say. Someone may think you are completely misguided dangerous idiot, but by god he will defend your right to say it.
In the U.S 90% of people will tell you the U.S.A is the greatest country in the world, however under 15% of Americans hold a passport. In the UK everyone just simply wants to live in another country. We bitch about immigration, but we have it sweet compared with every Mexican border state in the U.S. California is one of the biggest economies in the world, but it is technically bankrupt and governed by Arnold Schwarzenegger. Fondly known as The Governator - note the irony and humour. Only in America could a foreign born movie star become the governor of state.
I love this country. It bemuses me, makes me welcome and has variation that blows my mind.
So, yeah... lots of cultural differences. I don't think humour is one of them, i certainly don't believe uptightness levels are differant, if anything the U.S is a more relaxed country as a whole... it has to be with so much crazy shit going on

Beware the Ninja Monkey.