SmokingWalking down the street while smoking is one of the biggest no-nos in Japan. In many areas it's illegal.
There's a practical reason. Japan is a highly urban society. If you're walking with a cigarette there is a good chance to burn someone or blow smoke in someone's face.
Like many of Japan's rules of manners and etiquette — it makes good sense. If you live in Japan long enough you'll wish every country had this rule.
Food and DrinksThere are excellent vending machines and convenience stores everywhere in Japan.
In the old days, it was considered rude to walk while drinking or eating something. It's still considered to be a low class thing to do. You'll still see many people buying a drink from a machine — then drinking the entire drink while standing beside the machine.
Walking and TalkingLoud public conversations on mobile devices are rude in Japan. Walking down the street while loudly gabbing on the phone isn't cool in Japan. It will annoy people. If you do need to talk on the phone it's best to be quiet and discrete. Stand off in a corner somewhere and keep your voice down.
It also might save you from being hit by a truck.