From Print Book to eBook

The technology of book design has come a long way from the days of paste-up and Letraset.
Book designers and typesetters now use sophisticated software for page layout and combining text with image on a page is made easy with the drag and drop toolsets. The problem comes when design studios are asked to produce eBook versions of their ready-for-print layouts.
What changes are required to the format and typography on the page before a successful eBook version can be delivered?