Draft Comments are Enabled but not Saving on the New Comment Page
You have enabled the Draft Comments feature on the Email to Case Premium Outbound Configuration page, but notice that drafts are not saving on the New Comment page.
This issue is likely due to user profile permissions on the Case Comment Draft object.
The Case Comment Draft object comes packaged with 3 custom fields that Email to Case Premium users will require access to:
Permission sets can be assigned to end users by navigating to Setup | Administer | Manage Users | Permission Sets. From there, select the "E2CP End User" permission set, click the Manage Assignments button and then click Add Assignments to extend the permission set to specific users.