It would be great to see for CiviMember and CiviEvent the option to have an email subject in the configuration section. And, similarly to how Drupal allows you, the ability to plug in variables here that will give detailed information in the subject. Variable examples could be First Name, Last Name, Organization name. So, if a membership form is using a custom profile that includes any of these fields, then one could plug in the appropriate variables so that the information would come across in the Email subject – "%First %Last has registered for %eventName". If a user does not have any custom profiles they are including, they could still at least have the option to edit the default subject if they want.