In Japan, 7-Eleven is also a popular fixture in the enormous convenience store (konbini) marketplace, competing with Lawson Station, am/pm, FamilyMart, and many others. As in the U.S., all 7-Elevens in Japan are open 24/7. gn0sis tells me that 7-Eleven's Japanese division is actually larger than its U.S. counterpart.
Things You Can't Find in the U.S.
You can buy hard liquor, including sake, at all 7-Eleven stores in Japan, while some states in the U.S. restrict or prohibit the sale of hard liquor at convenience stores.
7-Eleven stores in Japan sell manga, anime, video games, and DVD movies, catering to the large Japanese market for these products. They also sell paperback books, but I believe you can buy those in the U.S. as well.
Lunch boxes, or bento, are sold at 7-Elevens in Japan for people to eat at work. They typically include either sushi, some kind of noodles, or a fried cutlet over rice with some Japanese pickles. They are sold cold and meant to be eaten cold, but are a less expensive alternative to a noodle shop. A few breakfast and lunch items with Western roots are sold in Japan, including donuts and sandwiches.
Various hot foods, including corn dogs (!) and oden, are sold at the counter.
All 7-Eleven stores have a kiosk at which you can buy tickets to concerts, sporting events, and other attractions. The kiosk also lets you order prints of photos from a digital camera memory card, charge your mobile phone for a few minutes, and perform all sorts of other neat tasks that I didn't try out for fear of keeping someone waiting.
All 7-Eleven stores that I saw in Japan have a photocopier, while I have seen very few stores so equipped in the U.S. The cost is about the same: 10-20 yen (8 to 17 cents) per black-and-white copy.
Most 7-Eleven stores stock at least some kind of PC gear, including CD-R and CD-RW blanks, floppy disks, MO discs (similar to Zip disks but higher in capacity), and blank MiniDiscs. These are sporadically appearing in the U.S., but are really popular among Tokyo's tech-savvy youth.
Summer gifts (ochuugen) are sold in 7-Elevens in case you have the need to send overpriced fruits to your social circle at 3:00 in the morning. This is very much a Japanese custom. The goods look to be decent in quality, although they are relatively inexpensive compared to the more lavish boutiques and department stores.
Obviously, various Japanese snacks including Pocky, Toppo, and Crispy Curry Pringles are sold in Japan and not the U.S. You can also buy Pocari Sweat in quantities of up to two liters, but Coca-Cola only goes up to 1.5 liters.
Things You Can't Find in Japan
Slurpees. The ambrosia of overheated and otherwise sugar-deprived people is completely absent in Japan. The closest thing available is shaved ice -- kakigori in Japanese, but most Americans would call it a snow cone. Kakigori is often sold at festivals and on the street for 200-300 yen (about US$1.70-$2.50) in the summertime, but you can't find it at 7-Eleven.
Big Gulps. Apparently Japanese people don't like to buy their soda in obscenely large paper cups. The Gulp (16 oz, 500 ml), Big Gulp (32 oz, about 1 liter), Super Big Gulp (44 oz, 1.3 liter) and Double Gulp (64 oz, 1.9 liter) are not available in Japan, although you can buy bottled drinks of that size in Japan anyway. The cost is substantially higher: while a Double Gulp will cost you about $1.20 including tax in the U.S., a 2-liter bottle of Pocari Sweat might cost three times as much in Tokyo.
Big Bite Hot Dogs. Although you can try a heat-lamp corn dog in Japan, you won't find any of the Oscar Meyer rollergrill treats known as Big Bites at Japanese 7-Elevens. Your loss, I guess.
None of the 7-Elevens I saw in Japan sell lottery tickets, while nearly all in the U.S. do.
I don't recall seeing hot coffee at 7-Elevens in Japan, although more than 20 iced coffee and coffee-flavored milk drinks are sold. The U.S. has a growing bottled coffee market, but in Japan the market is enormous.
I never felt out of place in a Japanese 7-Eleven, although it is an American chain. The stores are very Japanese, even blasting the company jingle (Sebun irebun! Ii kibun! or 7-Eleven! Good Feeling!1) incessantly over the stereo system. The employees are every bit as lackadasical in Tokyo as in the states, due to serving impatient salary men all day long. They even have a little charity box where I can deposit my worthless 1-yen coins that I get in change.
Sebun irebun! Ii kibun!
Thanks to gn0sis
for correcting this.