One problem is that the people who are deciding whether SOPA is happening or not, don't have the experience needed. The Internet is a complicated place and people with little computer experience should not decide how to regulate the Internet. Shouldn't companies like Google know what's best for the future of the Internet?
But the biggest problem is that SOPA requires service providers to block access to entire sites if a user on the site is accused of copyright infringement. The consequences might be that if a user of Twitter is linking to a website with copyrighted material - the entire Twitter.com may shut down. Google can't detect copyright infringement and currently relies on copyright holders to bring offending material to its attention. May Google shut down if SOPA is happening?
Edit: The debate continues when the Congress returns after the winter holiday.
Source: The Washington Post, Wikipedia