Problem: If you delete the master branch of your project, people will get a git warning when they clone your project:
warning: remote HEAD refers to nonexistent ref, unable to checkout.
In addition, the cloned project folder will be empty which is a huge usability WTF.
Proposed solution: Add an option to the administration pages of a project where maintainers can select a default branch. Github does it this way.
I think the default branch is defined in the file .git/HEAD of a project repository, unfortunately it cannot be pushed to the drupal.org servers.
It's available http://drupal.org/node/1127732