I think that's the key - an idea where a single user can benefit immediately when they sign up. Put simply, "tools."
In a recent article, Joel Gascoigne argues an interesting point: focus on product ideas that are not social. It's natural for people to get excited by the social aspects of a business at the time when Twitter and Facebook have taken off. Most of us spend so much time on these platforms that many of our product ideas for new apps stem from social interactions. However there are some major problems with creating a business around social networks:
Social takes longer to validate that the problem is something people want. The product/service is not as useful when there are fewer users (network effect). Revenue usually comes after some kind of "tipping point" which is hard to specify.
The reason a tool is a better idea for a business is the fact that you don't need other users for a single user to sign up and receive a benefit from the system:
'To bootstrap you typically need a single user to be able to benefit from the service without having to connect with anyone else. That's Carbonmade. A user can sign up and create their own online portfolio without needing other people in the system to benefit from it.'
There are many social ideas that are great, but if you're the type of person that wants to build a healthy business sooner than later, you might want to build a tool which people find useful immediately.