Cite research studies or theories to back up your comments. Students seldom connect their online comments to specific course concepts because they don't realize they're expected to. Many times comments are unrelated to readings, theories or research topics discussed in class. Frame questions in terms of concepts. When posting a discussion question, ask students to answer it using specifics from readings.
Students are too nice on the Web.
Assign students to play out roles of devil's advocate, pessimist or optimist to help them take different sides and spur debate. Peer camaraderie is lacking. Assign online buddies. Pair up students to help each other troubleshoot software problems and respond to questions about course content. Instructors struggle to teach and not preach.