The Collaborative International Dictionary of English v.0.48:

main street \main street\ n.
   1. A main thoroughfare, usually one in a small town having
      the largest concentration of retail businesses.
      [PJC]

   2. Hence: (Fig.) The attitudes, environment, or life style of
      a small town; -- usually capitalized, and often used
      attributively; as, Main Street attitudes toward
      Washington.
      [PJC]

WordNet (r) 3.0 (2006):

main street
    n 1: street that serves as a principal thoroughfare for traffic
         in a town [syn: main street, high street]
    2: any small town (or the people who inhabit it); generally used
       to represent parochialism and materialism (after a novel by
       Sinclair Lewis); "Main Street will never vote for a liberal
       politician"