Be upfront about your app's costs -- yes, all of them. You are practically inviting legal issues when you shroud your app with hidden fees. Tacking on extra costs here and there is penny-pinching that could end up costing you big-time in court. Make sure the app clearly conveys all potential monetary charges -- both initial download and in-app options.
Ensure proper consents and disclosures regarding user data and privacy. First, identify what information from the user and the device will be collected and shared with others. Next, make sure that information is clearly disclosed and consented to before data is collected or shared. If there's a possibility that your app will be used by minors, disclosing your privacy policies is particularly important.
Follow applicable standards and regulations. Remember, if your app deals with kids' data, health data, or financial data, make sure that you're complying with relevant rules and regulations, which are more complex.
Understand the differences between mobile platforms. Each mobile operating system uses a different application programming interface (API), which includes different security features and permission handling, according to the Small Business Administration. So don't just assume one size fits all; adapt your code accordingly.
Above all, your app shouldn't have any unpleasant surprises from a user experience standpoint. Good luck!