Scratch your own itch
The easiest, most straightforward way to create a great product or service is to make something you want to use.... excerpt from Rework Book By Jason Fried & David Heinemeier Hansson.
That lets you deisgn what you know---and you'll figure immediately whether or not what you're making is any good.
At 77signals, we build products we need to run our own business. For example, we wanted a way to keep track of whom we talked to, what we said, and when we need to follow up next. So we created Highrise, our contact-management software. There was no need for focus groups, market studies, or middlemen. We had the itch, so we scratched it.
When you build a product or service, you make the call on hundreds of tiny decisions each day. If you're solving someone else's problem, you're constantly stabbing in the dark. When you solve your own problem, the light come on. You know exactly what the right answer is.