What makes an app successful?
This is the question that every developer (either an individual or a company) would like an answer to.
In order to see what makes the apps we all use on our smartphones so successful, we must take a look at them individually, and try to find a common denominator.
Now, there surely isn't one particular, magic factor that is guaranteed to make your app get into the top 10 downloaded apps of 2015. It is not a matter of: "Make your app icon purple" or "Integrate cute pictures of cats into your GUI" and your product will be downloaded by everyone, everywhere, immediately.
Let's take a fairly successful app and look at what makes it so successful. Viber.
(For those of you who aren't familiar with it, Viber is a VoIP and instant messaging app, which shares similarities with WhatsUp and Skype.)
Why is this app successful (apart from the purple icon)? There is one simple reason for this: They have eliminated the need for registering an account, or adding contacts. You install your app, and you are ready to phone the people whose contacts you already have in your address book (given that they have the app installed as well). They even integrate it into the default address book of your phone, so you get a small Viber icon next to the contact, telling you that you can use Viber to contact this person.
(Getting short on space)
In conclusion, Viber offers simplicity, utility, full integration, and minimizes the effort required on the user's part.