Check out this article from TreeHugger.com. In Feng Shui tradition removing your shoes at the door represents leaving worldly goods behind, but these are some other great reasons to go shoeless!
These "shoe off" and "door mat" practices came about to reduce the amount of dirt that gets tracked into your home. Shoes clearly can be a source of substantial dirt, as is evident from the footprints left when you walk in shoes on a clean, wet floor. Not only can shoes bring dirt into your home, but they can also bring pesticides, lead, and other chemicals that can be present in dirt. The U.S. Environmental Protection Agency (EPA), for example, showed that people bring lawn pesticides into their homes on their shoes. These pesticide-laden shoes were a major source of pesticide exposures, especially for young children who spend a lot of time on the floor and who put dirty fingers, dust, and toys in their mouths. Somewhat surprising was that the study showed that indoor shoe-wearing was a larger source of children’s pesticide exposures than eating non-organic fruits and vegetables. Taking your shoes off and wiping your feet on a door mat or other carpeting before entering were found to be important pollutant reduction measures.