Jon Brodkin from Ars Technica:

As Boucher explained, while PIPA and SOPA differ in some ways, they both would give the government ability to designate rogue websites, remove those sites from the Internet's domain name system, require search engines to remove the sites from results, prevent advertisers from doing business with the "rogue" sites, and lessen protections currently provided to site owners during Digital Millennium Copyright Act (DMCA) takedown processes.

Ars has had some great coverage on SOPA and why it really doesn't make sense. None of the significant opponents are saying online piracy shouldn't be combatted. They're just saying that giving huge content producers the ability to effectively remove American access to internet sites just because some of the content infringes or, worse, some of it might infringe is a surefire way to cripple innovation on the internet and the ability for America to participate in the ever-increasing online economy.