Whole-class discussions, in which students share and refine ideas with others, can now take place in online forums. In face-to-face classrooms, previous research has identified conversational techniques, such as building on ideas or providing reasoning, that can open dialogic space and promote dialogic discussions. However, few studies have examined how students’ collaboration can be enabled and constrained by the design of online forums. This article addresses examples of online discussions from multiple contexts to explore the benefits and challenges of different online forum designs, evaluating the success of each example based not only on conversational techniques but also on the design of the online forum. Based on each analysis, tips for teachers suggest how to navigate the potentials and pitfalls of online discussions.