Most of the ideas for where we want to end up (Either using a pre-processor like Sass or a post-processor like Autoprefixer) depend upon our CSS not being in a single file with over 13k lines.
Something like this needs to be done early in a release cycle since it will 1) invalidate patches that are currently based on a single wp-admin.css file (though MP6 already invalidated almost all of them from before 3.8) and 2) creating a patch like this is time consuming and relies upon wp-admin.css not changing while the patch is being created.
I’ve created ticket number #26669 and uploaded a patch which shows the grunt and script-loader changes necessary. These changes make it so the only difference you see when running WordPress from /src are that you have more css files being loaded in the browser. It will still work smooth.
Please share your thoughts on splitting wp-admin.css up. Ideally we can make a decision on this during or before the January 8 Dev chat in order to get this in for 3.9.