Different disciplines have studied the content of online user comments in various contexts, using manual qualitative/quantitative or (semi-)automated approaches. The broad spectrum and disciplinary divides make it difficult to grasp an overview of those aspects which have already been examined, e.g. to identify findings related to one's own research, recommendable methodological approaches, and under-researched topics. We introduce a systematic literature review concerning content analyses of user comments in a journalistic context. Our review covers 192 papers identified through a systematic search focussing on communication studies and computer science. We find that research predominantly concentrates on the comment sections of Anglo-American newspaper brands and on aspects like hate speech, general incivility, or users' opinions on specific issues, while disregarding media from other parts of the world, comments in social media, propaganda, and constructive comments. From our results we derive a research agenda that addresses research gaps and also highlights potentials for automating analyses as well as for cooperation across disciplines.