Like Ade I am a big believer in eating your own dog food (will you gain significant value using this product yourself) and Jason, get started knowing that what you may end up with may be completely different. Having worked on a number of projects over the years I also learned a lot about looking for popular ideas that were still being done in old ways.
Our current company came about because of all of the above reasons:
We saw a very popular concept in strategic planning (research by Bain & Company and others had shown that strategic planning was the most popular approach for business improvement year over year)
We ourselves had worked on many strategic plans but had always found it a struggle to manage because of the collaboration issue and the complexity of understanding what to do at any given time
The strategic planning software space was still largely being done with downloaded desktop software or written plans that tended to reside on a bookshelf (in the SME and SMB space) making it very un-collaborative.
So we made the connection and created a solution for the startup, SMB, SME market for implementing strategic plans in an online environment focused on action items that would walk people through the execution of the plan.
In the end we have used our own software to launch the company and the product and we are constantly using that experience to make improvements in the software. We have even gone a step further and documented via our blog several key aspects of our plan and shared that with our users and readers.
So in summary the right idea for us to focus on was one where believed there was already a well documented market that was still behind the times in software offerings, one that nearly every company in existence would need at some time or another and one where we had direct experience in using the product to grow our own business.