This study is the first to compare the integrative complexity of online user comments across distinct democratic political systems and in discussion arenas with different primary use functions. Integrative complexity is a psycho-linguistic construct that is increasingly used by communication scholars to study the argumentative quality of political debate contributions. It captures the sophistication of online user comments in terms of differentiation and integration, mapping whether a post contains different aspects or viewpoints related to an issue and the extent to which it draws conceptual connections between these. This study investigates user contributions on the public role of religion and secularism in society between August 2015 and July 2016 from Australia, the United States, Germany, and Switzerland. In each country, it analyzes user posts from the (a) website comment sections and (b) public Facebook pages of mainstream news media, from the (c) Facebook pages of partisan collective actors and alternative media, and from (d) Twitter. Almost as many user contributions implicitly or explicitly differentiate various dimensions of or perspectives on an issue as express unidimensional, simplistic thoughts. Conceptual integration, however, is rare. The integrative complexity of online user comments is higher in consensus-oriented than in majoritarian democracies and in arenas that are used primarily for issue-driven, plural discussions rather than preference-driven, like-minded debates. This suggests that the accommodative public debate cultures of consensus-oriented political systems and interactions with individuals who hold different positions promote more argumentatively complex over simple online debate contributions.