I found an interesting article on requirements for virtual learning environments. 3 things were of particular interest.
#1 The concept of stretchtext. When a link is clicked, supplemental text is inserted (or removed) into the original text. I assume this could work for graphics as well. it allows for additional info without moving to another page and losing total focus and direction.
#2 The concept of a Virtual Student Clone. Students learn by teaching their virtual clone. Metrics can be derived from testing the clones learning. Really interesting concept.
#3 The importance of emotion for learning and how emotion can be included in a virtual environment.
(if the above link doesnt work — http://www2.iicm.edu/cguetl/papers/everything_virtual/everythink_virtual.pdf )