Data Driven Pages is awesome - it saves us so much time, and is such an improvement over MPS Atlas as it's now dynamic.
But one thing it still needs is the ability to create a key maps, and to output a hyperlinked multipage PDF so that users can click on a tile in the key map and go directly to the appropriate page in the PDF. This is a basic functionality of map books, and something I'm surprised ESRI hasn't accounted for yet. If I'm making a map book, people need to know how to navigate through it. And they do that either through an Index, or through a key map. I'd say key maps are WAY more intuitive.