I think one valuable component that is missing online is the community environment. I feel that it was a vital part of my success as a student to have a sense of "community" and to a lesser degree even a sense of "competition." Online students miss the opportunity to "hang out" and commiserate with other students in their classes and to share thoughts and ideas spontaneously.
When I did my mentoring for my online students I encouraged them to try to find other students in their area to meet up with or to form an online community of some kind. Since that time another instructor set up a Facebook site for them to utilize as a place to share thoughts and ideas. I still think a "realtime" exchange would be more valuable such as a chat room session.
However, I still feel that nothing replaces those relationships forged over coffee and cramming for finals.