Tell your idea to whomever will listen, and you'll get valuable market feedback before writing a single line of code.
Don't try to guess what it is people want and give it to them. Don't ask for a show of hands. Try your best to write what you like, what you think your friends would like and what you think your father would like and then cross your fingers... The most valuable thing you have is your own voice.
For technology companies, information about what people do online is extremely valuable - it can be used to sell targeted advertising or sold to data clearinghouses.
In the past, there hasn't been much reliable information about startups and small businesses available online. It's information that's really valuable, and it's information that people want to share.
Happiness statistics may be most valuable in smaller, local discussions. Understanding how different sorts of programs affect the well-being of citizens would be enormously helpful to a mayor choosing between building a new bridge or offering a tax cut.
The Marine Corps is some of the best acting training you could have. Having that responsibility for people's lives, suddenly time becomes a really valuable commodity and you want to make the most of it. And for acting, you just have to do the work, just keep doing it.