A crucial prerequisite for the success of Model Driven Engineering (MDE) is the seamless exchange of models between different modeling tools demanding for mappings between tool-specific metamodels. Thereby the resolution of heterogeneities between these tool-specific metamodels is a ubiquitous problem representing the key challenge. Nevertheless, there is no comprehensive classification of potential heterogeneities available in the domain of MDE. This hinders the specification of a comprehensive benchmark explicating requirements wrt. expressivity of mapping tools, which provide reusable components for resolving these heterogeneities. Therefore, we propose a feature-based classification of heterogeneities, which accordingly adapts and extends existing classifications. This feature-based classification builds the basis for a mapping benchmark, thereby providing a comprehensive set of requirements concerning expressivity of dedicated mapping tools. In this paper a first set of benchmark examples is presented by means of metamodels and conforming models acting as an evaluation suite for mapping tools.