Ask HN: How is Pinterest different from a Tumblr with a masonry theme?

I understand Tumblr doesn't necessarily encourage creating a new account for every 'interest'. But if/when they do, how will Pinterest differ?

From Pin Etiquette: "Pinterest is special because of the people who use it."

Well technically, MySpace is special because of people who use it too. Is there anything beside the focus on interests that makes this product special?

edit:// I haven't tried the iOS app, but there's no mention of being able to pin things in MobileSafari.