Just found this thread...
I can only suggest stuff to bring from the US, but you'll get the idea.
I found that eco bags are really useful in Japan and they can be great low-cost souveniers. I hear many expats stock up a bunch of bags from Trader Joe's or Whole Foods to bring back to Japan. Basic canvas bags at TJ's are about $2.99 and they're not sold in Japan (that's the key). Many bookstores have their own totes, as well as department stores and museum shops.
As far as I know, brands like Old Navy and Ann Taylor don't have stores in Japan. Bath & Body Works is a great place to stock up on lotions and soaps when they're on sale. They are not sold in Japan. It seems like that Japanese prefer fruity scents (esp Orange-y ones). Preserves from Sarabeth's Kitchen are great, but you need to pack carefully. My sisters said they'd never seen microwave popcorns when I brought one. Just make sure that you give accurate instructions as Japanese microwave ovens don't have the preset "popcorn" buttons!
My mother liked ragluhs (the spelling may vary) that I got from Ralphs. Many Kosher bakeries have pastries and cookies that aren't sold in Japan and they're not too sweet. If you can get a hold of premium chocolates (like Joseph Schmidt), they can be nice, too. Miniature-size candy bars that are sold in bulk at Target, etc., are fun too. They might be on the sweet side, but because they're small, your Japanese hosts may be able to enjoy them. Holiday versions of M&M's are fun too.
Coffee table books on your home turf may be nice to give, but they can be a bit heavy.
Spices and condiments that you know how to use. Even a box of Lipton onion soup mix can be good. Mix with sour cream and you get a dip! Taste of America! Chilli mix (like Wick Fowler's 2-Alarm Chili Mix) or BBQ sauces might be nice too. Chili goes really well with white rice. You should be able to find other ingredients (tomato sauce, meat, etc) in the local markets in Japan.
If you're in the States, shirts and caps of baseball teams with Japanese players would be nice, especially if you have one of those teams in your town. I saw some photos of Kobe Bryant while in Japan. Lakers still sucks since Shaq left, though. If you know the person you're bringing souveniers to is a big sports fan, you may want to ask beforehand so that you can bring in something that's one of a kind.
But as always, local craft items, like dolls and the like, are nice. Don't bring anything huge, though. Remember that Japanese homes are usually tiny!
Maybe I'll come up with more later....
