[Coda]

Well, the first thing to observe is that SOPA is only capable of targeting non-US websites that serve US users. This makes Craigslist immune, as well as Wikipedia and YouTube. Coincidentally, since The Pirate Bay is a .com site it's also considered to be a domestic site (based on the definitions ilsted in section 101) and is immune to SOPA.

Second, section 102 is in the hands of the Attorney General, not the rightsholder. This means that the only way to have SOPA brought to bear is to convince the AG that it's worth his time. Unfortunately, it also gives the AG unilateral, unchecked power to block any foreign site, because there aren't sufficient regulations on who the AG is allowed to target.

Section 103 is the scarier part. This one allows a rightsholder to raise a claim against any non-US site that it feels has harmed it under some frighteningly broad conditions. The operator of the site can either do nothing and accept the block, or raise a counter-claim within five days. But if they raise a counter-claim, they have to submit to US law. And in doing so, they open themselves to a lawsuit for copyright infringement, and according to SOPA they have to pay the attorney's fees of the plaintiff if they're found to be guilty (and the only thing necessary to establish guilt is to allow people to post stuff, because someone might post a link).

And then section 201 is the part that actually affects US residents because it redefines some aspects of copyright infringement. This is the part that says "if a song is worth $1, and you sing the song and post it on YouTube and get 2500 views, you're now a felon because $2500 is more than the threshold for misdemeanor infringement." This is also the part that makes you liable for damages if you used something under what you believed was Fair Use or some other legal protection but were wrong about your belief -- under DMCA, being wrong just meant you had to take it down and maybe pay some damages if you made money on it; under SOPA, you would have a permanent criminal record that would bar you from getting most jobs for a very long time, even if you didn't make any money. And a real danger here would be that a website operator might be found complicit in the infringement by providing the service, meaning the website operator would ALSO be a criminal.

TL;DR: If you're an American it's actually not as bad as the hype would indicate (if you're not American then you'd better get yourself a .com address right now, what a handy loophole), but it's still a terrible piece of legislation just because of how badly it's written and how poorly defined the actual offenses are.

Edit: If SOPA passes, a useful challenge might be to appeal to the Eighth Amendment, to consider if the punishment would be considered "cruel and unusual" relative to the crime you've been accused of.