1. Define a previously unrecognized niche

2. Strong ability to leverage natural search as the primary means of user acquisition

3. Service that empowers people to make a living

4. Free (or near free) alternative for a previously high cost service

5. True viral distribution potential

6. Ability to jumpstart user acquisition through distribution partnerships

7. Story that lends itself to mainstream PR

Yup, seems that the author got it right. More details here.