Identifying requirements, making prototypes, being responsible for the full life-cycle of a set of features: code generation to create sample mobile apps; executing user's code on server side with security restrictions; billing subsystem to support plans and add-ons, limits and notifications.
Supporting a variety of services, including authentication, data and file storage, messaging (publish-subscribe and push notification). Code refactorings and improvements (e.g. integrating JCache standard into existing architecture with an ability to experiment with implementations), designing scalable REST APIs.
There's also a client library for Android and Java, where I've been active contributing. It is open-source on GitHub: https://github.com/Backendless/Android-SDK