[dirtyroboto]

Tatami are very expensive and very delicate. You should not even wear slippers on tatami.
But before you can even make that mistake you will leave your shoes at the door. Man, you can stick 100 chopsticks in your rice and you won't even get close to the offence you will cause by keeping your shoes on. You would cause less offence by arriving naked!

Footware is a part of the earth elemental and is not allowed into the house beyond the front door. This space (where you find shoes) is considered a meeting point between outside and inside. The inside of a persons house/apartment/room is required to be clean (spiritually) less you invite bad energy upon yourself, Shoe's can pick up all sorts of bad shit (energy) and transfer this to your lovely space. Wood and straw (tatami) are very good at taking this bad energy up, and as most Japanese houses have woodern flooring and sometimes tatami it is required that you not contaminate a persons home with bad energy.