Today's Web users are faced with a large number of available online communities for every domain. While there are rules-of-thumb for the choice of a specific community, the validity of these heuristics has hardly been tested empirically. Furthermore, there is a lack of well-founded measures that allow for a systematic comparison of different online forum communities. In this contribution, we propose the concept of discourse quality as a means to this end. This measure is conceptualized from a user perspective and combines quantitative as well as qualitative parts, including a codebook for content analysis. To show the applicability and the usefulness of this measure, we systematically compare 34 online forums with varying degrees of membership size. We are able to show that the forums with the most members online consistently show high discourse quality. Finally, we discuss the potential of benchmark measures for future online community research.