Publish-subscribe is a powerful paradigm for distributed communication based on decoupled producers and consumers of information. Its event-driven nature makes it very appealing for large-scale data dissemination infrastructures. Various architectures were proposed in recent years that provide very diverse features. However, there are few well-defined metrics in the publish-subscribe area that would allow their evaluation and comparison. In this paper, we provide a broad overview of relevant quality-of-service metrics and describe their specific meaning in the context of distributed and decentralized publish-subscribe systems. Our goal is to provide a common base for future evaluations of emerging systems and for the design of qualityof- service aware publish-subscribe infrastructures.