No Eating While Walking
For a New Yorker, this one might be difficult to grasp, but here it is. You should avoid eating while walking. While this is not so uncommon in the West, it is truly a faux pas in Japan. If you do so in Japan, you will get some odd looks. You can eat on the train or bus. Doing so on long-distance express trains is the biggest no-no.
There are a few exceptions to this rule, however, as eating ice-cream on the street is not seen as rude or sloppy. This holds, even if it is one of the messiest foods on the face of the planet. One final note -it is also not considered impolite to drink standing next to a vending machine.