Service-oriented software systems (SOSS) are becoming the leading paradigm of software engineering. The crucial elements of the requirements specification of SOSSs are discussed as well as the relation between the requirements specification and the architecture of SOSS. It is preferable to understand service orientation not to be limited to Web services and Internet only. It is shown that there are several variants of SOSS having different application domains, different user properties, different development processes, and different software engineering properties. The conditions implying advantageous user properties of SOSS are presented. The conditions are user-oriented interfaces of services, the application of peer-to-peer philosophy, and the combination of different technologies of communication between services (seemingly the obsolete ones inclusive), and autonomy of the services. These conditions imply excellent software engineering properties of SOSSs as well. Service orientation promises to open the way to the software as a proper engineering product.