As drafted, the legislation would grant the government and private parties unprecedented power to interfere with the Internet's domain name system (DNS). The government would be able to force ISPs and search engines to redirect or dump users' attempts to reach certain websites' URLs. In response, third parties will woo average users to alternative servers that offer access to the entire Internet (not just the newly censored U.S. version), which will create new computer security vulnerabilities as the reliability and universality of the DNS evaporates.
It gets worse: Under SOPA's provisions, service providers (including hosting services) would be under new pressure to monitor and police their users’ activities. While PROTECT-IP targeted sites “dedicated to infringing activities,” SOPA targets websites that simply don’t do enough to track and police infringement (and it is not at all clear what would be enough). And it creates new powers to shut down folks who provide tools to help users get access to the Internet the rest of the world sees (not just the “U.S. authorized version”).