We respond by notifying the person who posted the material, giving them a chance to make a case for why the material is non-infringing.

The important thing about DMCA is that if we follow this fairly decent procedure, we, as a website, are legally protected from the claim that we contributed to the copyright violation.

The SOPA dramatically alters the careful balance in favor of “alleged” copyright holders. It makes it impossible for websites to find a reasonable safe harbor allowing them to continue to host user-generated content. And that is life-threatening for websites like Stack Exchange.