Nothing more important than retention. Even more important than getting the users in the first place.

Nice article on how to keep them in your app. 6 tips:

  1. Performance - work smoothly, run quickly.
  2. Design - easy to use, easy to understand.
  3. Same experience on all devices.
  4. Usage - don't drain the battery, don't get to many permossions.
  5. Murphy's law - make sure all your funcionalities are working!
  6. Play nice with other apps - refer users to them, work with them.

Overall, this should give any developer a general idea how his app should behave, no matter the type of app it is.

Full link here:
http://readwrite.com/2015/02/02/app-developer-checklist-features-performance