DNS blacklisting as a concept is simply unacceptable. The way it is applied in SOPA requires that a certain classification of DNS caching servers must pretend like a website does not exist, which reduces trust in domestic caching servers and stifles DNSSEC.
If people move to foreign DNS (or local caching servers), which is incredibly easy, the entire thing is circumvented. But a tricky side-effect takes place: the system is now balkanized. Servers in the U.S. believe the naming system to resolve to one IP, and servers everywhere else resolve to something completely different.
This balkanizing effect can break the effectiveness of CDNs, can congest Internet traffic, and overall reduces the credibility of the naming system. Passing SOPA would mean other countries would follow in the same footsteps. Once every country believes the delegation chain can resolve to whatever they want to, what is the point in an international naming system?
These are only some of the problems with DNS blacklisting -- nevermind the security problems -- which are not worth it especially considering it is so easily circumvented.
Additional problems with the bill, such as the vague wording which may consider Tor a tool for "circumventing" DNS blacklists and, therefore, illegal, demonstrate a huge lack of forward thinking and an unsustainable approach to copyright enforcement.
The doomsday scenario is that government sticks its foot in Internet policy and communication and pretends it actually has the reasonable capability to prevent piracy. It will never have that capability without large-scale violations of privacy.
There are also a good amount of talks regarding the precedent SOPA may have for general purpose computing and a whole host of other sensitive topics. I don't think it's responsible to just point at provisions in the bill and say "well it seems to add enough oversight".
Besides, the bill encourages preemptive takedowns by providing immunity. The tech industry behaviors that will result would be devastating.