I think there needs to be a clarification here, because many people don't realize this, especially women who have little exposure to pornography...
There is Playboy... and there is everything else. Playboy owns the market on softcore stuff, so no one else even bothers to try. Penthouse, Hustler, et. al. all compete with each other to distribute smut of the hardest quality.
Despite what is written above, Playboy is really nothing at all like Penthouse. It's closer to Maxim in content than other porno mags, with the key distinction being that people appear nude in Playboy. There are interviews with actors, stories about pro athletes, and reviews of movies, books, and albums. Basically Hugh's saying "We're gonna give you an interesting magazine that you can actually read, but we realize that you like seeing naked chicks, so that's what we're going to give you."
There is no sex in Playboy. Oh, there's the innuendo, the dirty cartoons, and the like, but this is softcore stuff. Women leaning against a piano naked, testing out the durability of a mattress naked, or reading Aristotle's Poetics naked. If you want some contrast, go to Playboy's website, and then Penthouse's. Or even better... show a girl a Playboy and a Penthouse side-by-side. Ask her which one is more offensive.
It is generally accepted that you can use a Playboy pictorial to launch your career (see: Pamela Anderson, Jenny McCarthy). Er, well, I guess you could call it a career. You can also put on a resumé that you were a columnist for Playboy and still be taken seriously.
So anyway, that's the difference.