Know Your Stuff
Understand the problem you are approaching to all its depths and that your solutions stands true to all the circumstances put forth.
Whether technical people, non-technical people, teenagers, or any other category of people the app is going to be useful for.
Whether Android, iOS, Windows, Symbian, Ubuntu etc.
The number of users to benefit are maximum
Compatibility and Interoperability with other applications
Native Apps, Web Apps or Hybrid Apps what suits best for you..