It'd be cool to extend bzr with some plugins to allow it to handle or show differences for specific file types in ways that are useful for those. For example, OpenOffice.org Writer files are binary blobs, and it's difficult to diff them at the moment. However, it would be possible to write something that extracts the text from the blob, and shows something like a wdiff.
This could be extended to all sorts of file formats. An image diff might generate a new image that colors changed parts with red. A source code diff might understand the language and work at the level of semantic language elements.