in reply to Email subject lines should be written:
Other: Write it before the body: it's a headline.
Your message may be important, but the Subject will determine when (and sometimes if) I read it. I always work with the preview pane off, so the Sender, Subject and Date may be the only part of your message I ever see. Of those, the Subject is the one you control for any given message, so make it count.
Once I've opened the message, the Subject doesn't really matter, other than to track and group the conversation, depending on the mail reader.