What is the significance of the classic Monopoly pieces?

Here's the official line straight from Hasbro, current copyright holder for Monopoly:

With the tokens serving as such an important part of the MONOPOLY game experience, it's hard to imagine the game without them. However, the first game produced in 1935 did not include tokens. Charles Darrow, who originally brought the game to Parker Brothers, recommended that players use household items like buttons and pennies to move around the board.

Parker Brothers decided to include mover-tokens in the game. The edition produced from 1935-36 included four small wax wood pieces, or pawns. In 1937 Parker Brothers, determined to preserve the game''s tradition, decided to produce die-cast metal tokens that were reminiscent of objects found in households across America. This decision marked the introduction of the flatiron, purse, lantern, car, thimble, shoe, top hat, and the rocking horse. The top hat was modeled after the chapeau of the game's Chairman of the Board, Mr. Monopoly, and the car, his 1930s roadster.

Two additional tokens, the battleship and the cannon, were also added in 1937. They were used at the time, in another Parker Brothers game called Conflict, which made it easy to add this pair to the MONOPOLY game. These 10 tokens--flatiron, purse, lantern, car, thimble, shoe, top hat, rocking horse, battleship and cannon--were used until 1942.

With World War II came a metal shortage in the United States. As a result, during the years 1943-47, MONOPOLY games contained tokens made out of wood. Shortly after the end of World War II the metal tokens were brought back to the game.

The early 1950s brought the addition of three new tokens and the departure of three original tokens. The lantern, purse and rocking horse were replaced by the dog (Mr. Monopoly's dog, Scotty), the wheelbarrow and the horse and rider. These three MONOPOLY game tokens still exist in the classic edition--the MONOPOLY game in the red and white box found today in stores and more than 200 million homes world wide.

So the tokens were not designed to signify anything by the game's designer--he wanted people to use tokens from around the house. Many people carry out this tradition, either by using coins for missing tokens or borrowing tokens from other games. Me? I'm always the car.