Building a Product That Sells

the better mouse trap myth
basic rules to follow
build it for yourself

Let's start with the product because after all that is what your customers will want and what will define all of your future efforts, regardless how lofty your philosophy is. 
We need to start with dispatching away of the widely quoted statement “Build a better mousetrap and people will beat a path to your door.” — On rare occasions it's true, especially for social networking apps like Facebook, or great stuff like Pandora, but most of the time engineers create technological marvels that fail to catch on public attention. 
There is no single formula for success, but at least I wrote down a few characteristics of a successful application:

performs the advertised function very well
it's simple to use according to “Don’t make me think” rule
it's an eye-candy, designed for a specific demographic group
it's well designed for the platform it runs on (phone, tablets)
induces people to share stuff via social networks
it's easy to find in app store search
provides reasons to use it frequently
reminds of itself via unoffensive notifications

First of all your product should be useful, does not matter if it is a game or a scientific calculator, but it should do what it is designed for very well. The product should be very easy to use; any complicated options, confusing messaging, wrong assumptions, functions that take too long to execute are a sure way to loose users. Product should be designed well taking in consideration your target demographic. Your product should be designed in such way as to encourage user to share the experience, quotes, images, likes and comments via your product. Product might remind user of the new content available via notifications. Social sharing is the fastest and easiest way to reach a larger audience. Product should be named well so people can find it, it should provide a good description, if possible in multiple languages.
Finally, it is always a good idea to build a product that carters to your own needs, knowledge and interests. People often fall into a trap of developing a product based on current fads and fashions, development of a good product is usually a long process and when you are not passionate about it, you probably will fall short of success.

Uki's Google Java Technology blog