When you are sending an email to a group and you are using a template, the email subject will not be included in the email template. For example if you add a subject to the 'Standard email' template then choose a group (eg: US East Contacts) and choose 'New Email' from the actions list on the left hand side, the subject field will not populate after you choose your template:

Group template issue

When sending an email to a group the subject field is not populated if you select an email template.

This issue has been resolved in Sage CRM 7.3c Hotfix 3.