I think culture shock is usually considered to be something somebody visiting or living in a foreign country can't deal with.  So far I haven't experienced anything of that nature.  Still there are many things that seem to be different between Japan and America. 

Sometimes a friend may tell me things are different here but it might not really be true.  For example, one person might visit America and meet some very friendly people, go back to their home country and tell everybody that the people in America are very friendly.  Another person might get mugged and go back and tell everybody that America is full of mean people.  My point is the observations below might not actually be true because I only have my experience to draw on.

I'll give you the number one mistake I think American's generally make about Japan.   It is that all Japanese women are "easy", especially for foreigners.   The reason visitors make this mistake is that they go to Roppongi, a city where there are many women who's hobby it is to sleep with a new man every night.   There are many foreign visitors to Roppongi.  Those visitors go back home with the impression that all Japanese women are like the women who hangout in Roppongi and of course this is just not true.  That would be similar to judging all people in Los Angeles by the people you meet on Venice Beach or Melrose Blvd.

So, here are some of the differences I've noticed so far.

• Blowing your nose in public is considered extremely rude.  Instead you are suppose to sniff and snort until you can get to a bathroom or someplace private. This is pretty much the opposite of how I was raised. In fact my parents used to get pretty angry if I kept snorting as a child. It's a little difficult to ignore all the snorting you hear.
  
• Making noise while you eat is okay. Slurping, chewing and talking with food in your mouth. All these things are fine and excepted in Japan. Or so that's been my experience so far.
  
• Barfing. While not acceptable, because so many people go out drinking at night and because they all take the train home I see a pile of vomit at least once a week somewhere on my way home from work. I've seen more vomit in public in Japan in the last few months than I've seen in my whole life in America.
  
• Spitting. I'm not sure how acceptable this is supposed to be either but I see lots and lots of people spitting. Everywhere! They spit on streets, on sidewalks, on tiled floors inside stores you name it. Yuck!
  
• Naked people on television and magazines. You see alot more naked people on Television, especially late at night, and also in magazines and not just porn magazines. For example a news magazine might have 10 pages out of 200 or so of naked women. Another thing you see is in TV dramas (read: soap operas) you see people having sex. It's not explicit but it's usually noisy. They do keep it short though.
  
  This is one of those topics where many people like to claim that America is prudish compared to the rest of the world. Well I'd like to point out that although you can see naked people on TV in many countries, America is considered the PORN capital of the world and most cable companies carry 1 to 3 24hr porn channels into most American houses. I'd say that probably puts America at the top of the list of explicitness.
  

Other Differences:

• Clothes sizes. Usually most stores only carry up to large size shirts. No extra-large. Also shoe sizes generally stop at around size 8. If you need a larger size you'll probably have to go to specialty store.
  
• No unlimited local phone service. All calls, even to the person next door, are charged by the minute. Oh, and signing up for phone service costs about $750.00. The weird thing is you can buy somebody else's phone line "used" if you don't want to pay the $750.00. In other words, when I leave Japan I could sell my phone line to somebody else for any price I can get. I don't understand why the phone company charges you $750 to press buttons on their computers to start your new service but if you sell your service to somebody else, even though they have to press the same 5 buttons to "move" it to another place, they don't charge anything (or very little)
  
• No returns.  If you buy something and you open it it's yours.  Many larger stores in America will let you return almost anything within a certain time period as long as you have a receipt.  Not it Japan.  This came as quite a shock to me.  I bought a digital camera, brought it home, tried it out, didn't like it, wanted to take it back, couldn't.  Back in the states this would not be a problem.
  
• You can rent music CDs.  Although it might not be a huge industry, there is chain of very large video/cd rental stores, Tsutaya, and they have lots of music CDs to rent.  On the other hand the average music CD in Japan costs $32.00  CDs from America are often down around $18.00