Case in point: Posterous. As discussed more extensively by Dustin Curtis (link below), their novel feature was to allow blog posts by email. Any existing blogging platform could have added this feature. But Posterous is different; the product is posting-by-email.
I think something similar can be said about Twitter. Again, any social networking site could have added the option to send short text messages to their already large user base. Facebook in a sense encompassed Twitter with its News Feed.
But again, Twitter is different; short text messages are the product.
http://blog.dustincurtis.com/i-dont-laugh-at-startup-ideas-a...