Is this a problem that you personally have?
It's best if it's a problem that you personally have and are actually building the product so that you can use it yourself. However this doesn't mean that you don't have to do customer development/validation.
We built StartitUp because we were frustrated about all the startup knowledge and tips being too scattered and unorganized for us to efficiently utilize for our startup, so we wanted to organize it for ourselves. Therefore, while solving a problem for ourselves, we also solve the problem for many others like us.
Case Study: Craigslist, the biggest classifieds listing website in the world, was created not because Craig Newmark had this grand idea, but because he had some friends who asked him to build an webpage so that they could list the stuff they want to sell. Slowly people started using it because the only alternative was the newspaper. Craig wasn't especially good in the business of classifieds ads, but he still made it happen "accidentally".
This is what Craigslist looked like in the beginning. They haven't improved much since.
Case Study: Facebook was built because Mark Zuckerberg wanted to meet girls, not just because he thought of this great idea and thought that he could make money off it.
