In my humble opinion, there is only one factor that could make an app become a hit. It's the population will receive the app. It does not matter if it is well design or even have a solid structure running under the hood. Let me give you an example, here at my country, Brazil, a few months ago a app received more than 200K downloads in a few days. The app has only one Activity, and it is poorly programmed. All the flaws are clearly visible, but the app become a hit here. Most of my friends had the app installed because was fun making the weird noises that come out from the app. (The app name is Vem Monstro!). To sum up, there is not a key to success in the app world. Sometimes, you only need to have a good ideia, and timing.