Why don't we add a "Mobile First" tag to the Layout column of the Themes Directory's Tag Filter ?
Currently the 4th most popular WordPress Theme is Customizr which is a "Responsive" theme, still based upon Bootstrap 2.3.2.
Various NEW themes are popping up based upon the current "Mobile First" designs of Twitter Bootstrap and Zurb's Foundation.
The user experience between the newer versions of these frameworks and the older versions differ (particularly with the primary nav menus), and will result in users desiring one format over the other.
Why not separate the two versions NOW, using a Tag Filter which would allow end users to easily find "Mobile First" WordPress Themes that exclude the older design versions ?