Embedded graphics and `attached' sounds and graphics can liven up the material.
Hyperlinks allow keywords used thoughout a text to refer back to definitions or other related sections. This is very useful, and supplements the usual index.
Taking an existing document and marking it up into a minimal web document is relatively easy; adding graphics, sounds, animation and inline cross-references takes extra time.
I have started work on putting my OS2 notes in web format (from the LaTeX originals). Click here to see. Lawrie and Mike have being doing similar things with the CS1E Millieu material.
You can add annotations to a web document; these are stored locally and do not alter the original document. Currently, these can only be placed at the bottom of a document.
The only other problem is actually getting students to use the information. I guess this is a problem regardless of the medium used.