This page lists some types of sites that the Senate Judiciary Committee's September 2010 draft COICA bill might take offline, along with some reasoning:
One-click hosting websites such as Dropbox, MediaFire and Rapidshare: these sites allow users to upload anything, and do not police files unless they receive DMCA takedown notices. That's the way the law currently works, and although it causes problems, it at least strikes a balance between copyright enforcement and freedom for sites to innovate. Under COICA, the Department of Justice (DOJ) could decide that there is "too much" piracy on any of these sites and it is therefore "central" to their businesses.
MP3 blogs and mashup/remix music sites like SoundCloud, MashupTown and Hype Machine: the RIAA has a long history of litigating against unauthorized sampling and musical quotation in all of its forms; while these uses should be protected by the fair use doctrine, the DOJ might come to another conclusion, especially under pressure from the RIAA, and just take MP3 blogs and remix music sites offline whenever the recording industry complained about them.
Sites that discuss and advocate for P2P technology or for piracy, like pirate-party.us, p2pnet, InfoAnarchy, Slyck and ZeroPaid: while these sites contain a great deal of news and political speech, they also regularly link to tools and information intended for file sharing, and the DOJ could well decide that infringement is "central" to their purpose and take the entire sites offline. That outcome would be fundamentally contradictory to freedom of speech.