SOPA’s unit of analysis is a “site or portion thereof” See Section 102(a). By contrast, if a portion of a site is infringing, SOPA’s unit of censorship is everything at the domain name. See Section 101(17), 102(c)(2)(B) and 102(c)(2)(A).
The harm that does to ordinary, non-infringing users is best described via a hypothetical user: Abe. Abe has never even so much as breathed on a company’s copyright but he does many of the things typical of Internet users today. He stores the photos of his children, now three and six years old, online at PickUpShelf* so that he doesn’t have to worry about maintaining backups. He is a teacher and keeps copies of his classes accessible for his students via another service called SunStream that makes streaming audio and video easy. He engages frequently in conversation in several online communities and has developed a hard-won reputation and following on a discussion host called SpeakFree. And, of course, he has a blog called “Abe’s Truths” that is hosted on a site called NewLeaflet. He has never infringed on any copyright and each of the entities charged with enforcing SOPA know that he hasn’t.

There are millions of US internet users just like Abe. If you are one of them, I urge you to make your voice heard by going to AmericanCensorship.org or EngineAdvocacy.org/voice.
* All names of services meant to be fictitious.
Images are public domain, sourcing here.