Since you didn't specify who you were addressing (Japanese coming to America for instance or vice-versa, etc.), i'll interpret the question as an open-ended one and give you my thoughts on the question posed.
As an American who was neither born nor raised in the States and who has also had the opportunity to travel abroad extensively, I am no longer as impressionable as I once was. That being said, "culture shock" no longer impacts me in any extreme sense of the word per se.
Yes, I suppose Japan is unto itself like no other country in the world, and I would imagine had I lived a rather less-travelled or mundane existence, then I might have been more prone to some of it's influences or more surprised or even shocked at some of it's practices (eating roe for instance? WHAT?!?! We use that for bait here! etc.). That being said though, having been raised in an Asian household where my mother's native language was always in the background, "growing up Asian", etc., my experiences in Japan felt very comfortable to me, even though many were for the first time and I am not Japanese (although sometimes I feel like I am, heh).
A more distinct and sharply different "culture shock" came to me during my travels throughout India and Africa a few years ago. Witnessing such abject human poverty/disease/famine on such a massive scale firsthand puts your whole life in perspective in a "There but for the grace of God go I..." kind of way. All the little things we take for granted (myself included), material possessions we feel "we deserve", or are privileged enough to have like fancy cars, HDTV's, high-end PC systems, etc. ad nauseum, all amount to very little when put into proper perspective.
On a lighter note, probably the single biggest surprise for me re: Japan and "culture shock" was the fact that Japan has such a low overall crime rate for being such a densely populated nation with a cash-based society. It is not uncommon to see young kids nowadays walking around with thousands of yen (or considerably more) in their pockets unaccosted (something unheard of here in America, especially in the more urban metropolises). Having lived in both California (L.A.) and Florida (Miami) at one time, such practices are not commonplace (to say the least). Anyways, not really sure if that was the kind of answer you were looking for but that's my .02 cents.