I guess the vagueness of the language is why we have lawyers, but the way I read the first "or" clause, HN could be painted as infringing. It is primarily designed to offer services (link posting and comment discussion) that (happen to) enable copyright infringement (see the DVD key I posted above). It's not designed for the explicit purpose of enabling copyright infringement, but it
is designed to offer a service that, by its nature, enable and facilitates copyright infringement, if users were to use it for such.
To paraphrase:
the U.S.-directed site is primarily designed for the
purpose of offering services that enable
copyright infringement.
The way I'd expect a reasonable person to read that clause would be "you're a bad guy if you build a site that is specifically for distributing infringing content". The way I expect a lawyer would read that is "you're a bad guy if you build a site that includes functionality that can be used to distribute infringing content". Intent doesn't seem to be in play; only the fact that you offer a service that enables copyright infringement matters.
My understanding of it seems to be the thing that so many people have an issue with; the concern is that SOPA's loose language makes any content platform effectively illegal, giving the media cartels a sledgehammer that they can swing at just about anyone they please.