I like the idea of splitting drupal.css up into parts based on features they implement.... that you could enable or disable through the themes interface. maybe have selectable sets of css per functionality...
tabs - tabs-default.css, tabs-borderless.css, tabs-simple.css
fieldsets - fieldsets-collapsible.css, fieldsets-borderless.css, fieldsets-nolegend.css
icons - icons-modern.css, icons-kde.css, icons-gnome.css
ajax.css - ajax-enabled.css, ajax-disabled.css
colors.css fonts.css layout.css
opens up smaller and more user, friendly I think. quick disable of what you don't need, easy behavior changes... not sure if all those things are possible, but it would be nice.
This is very very good, and is similar to what the Civicspace theme does. --Jeff