I did that on a small scale, sort of accidentally, and dozens of people have emailed me with the equivalent of "Wow! That's cool!" It has also developed into a small, active community.
Although customer development is great and popular right now, what I did didn't use any customer development. It also wasn't originally intended as a business either, but your post doesn't mention anything about money :)
I took an existing web application and gave it a great user interface. That's it. If you can take an application that already exists and make it:
* 10X easier to use
* 10X more fun to use
* 3X faster to use
* Free
then you'll grow to thousands of users via word of mouth and your users will love you.
The easy part about it is choosing the application. Anything that you currently use that's cludgy can probably be improved a whole lot. The difficult part is innovating the user interface, but luckily that is also the fun part (at least it is for me).
This is also similar to what Gmail did. They made web-based email much easier, faster, and fun to use (while staying free), and that app probably has the largest number of appreciative users on the planet. Google didn't have to reinvent email to get those passionate users; they just took an existing thing and made it much better.