Why is the subject blank when replying to a post?
Subject of the reply is optional. If you were to reply from your email address, the subject most likely would be "RE:Subject of the original post" and that is what would display. If you reply to a forum post from the forum page, you can type the new subject but rarely ever you would have the need to change that information. When the subscription email is sent, the original subject will be listed in the email, so you can tell what is that reply in the reference to.
Hope that helps, please let us know if you have any more questions,
Supervisor, Customer Support.