This paper presents a short review of existing visualization systems that focus on online course discussion forums. Discussion forums in online courses are used as an important means for students-students and students-instructors communications. However, a large amount of asynchronous and heterogeneous posts in the discussion forums and their complex relationships make it difficult to track and extract meaningful information about the students' activities and progress in the online courses. Information visualization has shown promise on this to facilitate online learning, especially for predicting students' performance on tasks and assessments, understanding students' affects and sentiments, and predicting students' social behavior pattern and awareness. In this review, we detailed the applications of the existing visualization systems, their design considerations, and their strengths and weaknesses in the context of discussion forum analyses. This information will provide insight into how to build an effective visualization system for the online courses' discussion forums.