The recent increased use of Internet, social media, and networked business mark a development trend where software-based services flow to the open market for enabling service-oriented networked business. Our vision is that in future, organizations and individuals collaborate within open service ecosystems. An open service ecosystem is characterized especially by the autonomy of its entities, its evolution with respect to available services and collaboration types, and dynamic establishment of collaborations. For facilitating collaboration establishment in open service ecosystems features of services and cooperation facilities, and feature inter-dependencies need to be governed rigorously. Towards this purpose we have established a framework for unambiguous description of service ecosystem features. The framework comprises a conceptual model which provides especially a categorization of features, and a formalization of the conceptual model as a meta-model for service ecosystems. We show that the corresponding feature categories have their specific roles and semantics as part of different ecosystem elements and in different phases of service ecosystem processes.