I may be missing something but I noticed something that I think can be improved. Currently, it looks like if a developer wants to output a list using wp_nav_menu, they are stuck with unordered lists like this:
<ul> <li></li> <li></li> </ul>
What if the developer thinks that the appropriate markup should be a definition list:
<dl> <dt><dt> <dd></dd> <dd></dd> </dl>
Since there are only 3 kinds of lists in HTML, it seems that wp_nav_menu ought to have built-in support for
<dl> rather than relying on developers to write custom walker classes each time.