Drag & DropDrag & Drop is one of the essential elements of a good GUI experience, but it is often lacking in web GUIs. What makes D&D so effective is that it provides a direct action interface for the most common computing operation: copy.
For users, one of the most visible and useful features of O-O HTML is that it will enable D&D within and across web (and even desktop) applications.
As an example of where OOHTML D&D would be helpful, consider the numerous forms for entering contact information. Being able to drag from wherever a FOAF, vCard, or other contact item is displayed, and drop that into a form and have the various fields that match up get their value's copied, is what a decent GUI should do.
[#1] Even when a web application implements D&D, it rarely (ever?) will work across applications.