Imagine you are a shop owner and someone writes something offensive on the outside of your building without you even knowing about it. This bill would allow the government to shutdown your store before you even knew the writing existed.
Today we already have in place a mechanism to notify the store owner, but SOPA changes that. It makes it possible to shut down entrepenuers, the store owners and wealth creators on the internet, before they even have a chance to get started helping to revitalize the American economy.
A better base for the analogy might be a mall, since there are a variety of situations where SOPA could result in a slew of virtual storefronts being taken down due to the actions of a customer in a single store that wasn't doing proper policing of its customers (never mind whether the stores should be responsible for policing their customers, attacking this at the domain level is like shutting down a mall because of the nonviolent actions of a few customers, and blaming it on the mall.)