Status cues and other non-verbal communication are difficult to communicate via email, and thus often have a reduced impact. A more mundane advantage of the textual nature of email is that it provides a data trail of the discussion in a class. If you plan to give credit for class discussion, it is really possible in email to determine the rank order of frequency of contribution. In addition, an instructor can use the text of the comments to determine how well students are grasping the concepts in the class. For instance, a good discussion requires that students and faculty read their mail at least once a day, if not more.
Any lesser frequency makes even a simple discussion drag on for weeks.
If students must come to campus to use the system, their time is thereby limited. If there are not enough terminals or computers on campus access is further limited. In addition, some student will not have learned the basics of logging on and editing text. All these are hurdles between the student and mere access to the online discussion. Once students have access, they must still attend to the discussion regularly.