It's possible (and bad) to have no default location type

If you unset Default on the default location type, you're left with no default location type. If that happens, profiles using primary email silently fail to save an email address you enter. Likewise, Drupal user registration (possibly other CMSes--haven't checked) doesn't save the email to the created CiviCRM contact.

My proposed solution is to disable the "Default" checkbox when it's already active--you should go to another location and set that as default.

The only reason I think this is "minor" is that it takes someone actively going to the Location Types admin page and deciding that it would be a good idea to have no default.