D2D interactions are promoted both in the context of traffic and computation offloading. Applications with functionalities that are executed in parallel in more than one mobile device have been proposed while the idea of using another mobile device as a relay is going to be part of 5G. In this work, we first present the main types of D2D applications, then we discuss the characteristics of task offloading and the quality of experience in D2D ecosystems. After that, we list five popular questions regarding the ability of other users to be helpful in terms of resources, connectivity, availability, incentives, and security. To answer the listed questions, we use two mobile big data datasets, one experimental study, and arguments from our experience and the literature.