From http://americancensorship.org/

Website Blocking:

The government can order service providers to block websites for infringing links posted by any users.

Risk of Jail for Ordinary Users:

It becomes a felony with a potential 5 year sentence to stream a copyrighted work that would cost more than $2,500 to license, even if you are a totally noncommercial user, e.g. singing a pop song on Facebook.

Chaos for the Internet:

Thousands of sites that are legal under the DMCA would face new legal threats. People trying to keep the internet more secure wouldn't be able to rely on the integrity of the DNS system.