I find that while people put a lot of emphasis on making an "original" product, a lot of the more successful products tend to be clones. After all, if you come up with something completely original you need to do a lot of explaining to get people to understand it; Whereas if it's based on something they already know, it's much easier for them to comprehend.
There are a few metrics Pinterest has that others similar don't have.
In no particular order,
i) The name is a genius combination of the primary action, and the primary purpose behind it + it's highly brandable / memorable
ii) The facade of exclusive club via requiring an Invite / being able to Request an Invite
iii) They're scraping the web's content, and putting on their own site, thereby reducing / eliminating traffic leaks; I'm undecided if I think it's fair, something doesn't feel right about it - because where some may benefit from their content being copied (you could argue copied->stolen, and probably show damages), many more I believe will not benefit and those are merely feeding the Pinterest ecosystem.
... a few more that I understand but keeping to myself. :)
