Here is a good essay on determining what is a good idea for a startup. Might give some guidance on what you should build:

http://www.paulgraham.com/ideas.html

I recommend starting by understanding what you really want, and then see if others share your desire.

They want things to be easy.

read this great post: http://www.gobignetwork.com/wil/2007/1/5/forget-about-the-so...

The smartest entrepreneurs aren't the ones who figure out the solutions to big problems, they are the ones who actually understand the value of those big problems. Smart entrepreneurs focus on the very essence of why the problem exists and the pain it causes. Then, when they find a solution that works, they know how valuable the solution really is.

People don't know what they want.

if people knew what they wanted, the most successful companies would be the ones with the largest focus groups.

And even if they did, they wouldn't be able to explain it well enough for you to implement it.

People say they want "The Homer", but they buy...well, actually, sometimes they buy "The Hummer" which is two letters off, but very very similar in concept, style, and execution.