Due to the rapid growth in the number of mobile applications (apps) in the past few years, succeeding in mobile app markets has become ruthless. Online app markets, such as Google Play Store, let users rate apps on a five-star scale and leave feedback. Given the importance of high star-ratings to the success of an app, it is crucial to help developers find the key topics of user-reviews that are significantly related to star-ratings of a given category. Having considered the key topics of user-reviews, app developers can narrow down their effort to the user-reviews that matter to be addressed for receiving higher star-ratings. We study 4,193,549 user-reviews of 623 Android apps that were collected from Google Play Store in ten different categories. The results show that few key topics commonly exist across categories, and each category has a specific set of key topics. We also evaluated the identified key topics with respect to the changes that are made to each version of the apps for 19 months. We observed, for 77% of the apps, considering the key topics in the next versions shares a significant relationship with increases in star-ratings.